Being a cyclist, I always use to spend more money for my cycles, cycling gadgets and apparels. If you are not a cyclist, trust me, this is so usual among us. But my non-cyclist friends mostly think that I am a weirdo. People ask me why the heck I spend $1200 for just a pair of pedals. But only I, being the user of it knows that the pedal connects to my bike computer and measures the power exerted by my legs which is an effective metric for my training.
Everyone sees the world through their own filters. Those filters depends on their past experiences, the examples they've seen or what they tend to see in their day to day life. In my case, my non-cyclist friends would've seen people spending chunks of money to buy motor bikes, fancy phones or even jewelery. But they rarely see a person spending same amount of money to buy add-ons for cycles or even the cycle itself.
So, based on their thoughts and examples, people assume that others will also see the world in the same way as they do. Psychology has a name for this, False Consensus.
In psychology, the false-consensus effect or false-consensus bias is an attributional type of cognitive bias whereby people tend to overestimate the extent to which their opinions, beliefs, preferences, values, and habits are normal and typical of those of others (i.e., that others also think the same way that they do) - Wikipedia
How many times have we thought that our next door neighbor would've voted for the same candidate as we did?. Only the people in different part of the city, with different education background or from different socioeconomic class would've voted for the other candidate.
Here's another example. During college days, how many have thought that all our squad members wouldn't have prepared for today's test?. Then, when we get to know that one guy has prepared for the test, we think that he is acting weird or he is going against the party rules. Well, I am not sure about you, But I have been there and done that in most of my college life.
How false-consensus affects the product design?
The developers and designers come up with certain ideas and craft the UX of the product. They will usually test it within themselves and sometimes with a limited set of people inside the company. When they themselves or their colleagues like the interface, designers tend to assume that this idea is something that everyone would like. But when they present the product to the users or other members outside the team, mostly it turns out that others will not like the interface as the designers did.
You're not the user
The above quoted phrase is one of the hymns of UX design. You might be the top developer who has been writing complex code or a best product designer who has done many courses on product design. But you cannot be the user. You will not know how an user's mind will think, how their emotions are when they experience your product.
We decide the usability of our interface by examples, mostly by asking feedback from our colleagues after using the interface. Then we come to an assumption that this interface will be liked by the rest of the world as a bunch of people enjoyed it inside our office. Generalizing it based on examples are totally normal for humans and psychology has another fancy name for it: availability heuristic.
You tend to pick a decision based on the ease of retrieval of examples. Psychologists also says that the number of examples that comes to your mind drives your decision. Lets say, you have shown your decision to your team members and almost everyone in office. Most of them agrees that the interface is user friendly. But when any other team member gives suggestion or critics, the first thing that comes to your mind are these examples. You would automatically try to justify your design and convince yourself that most of the people liked it, so this is a good interface.
But hey, this is not wrong. All humans are driven by examples. It is most common among the people. For example, if you see a series of news events of child abduction, you assume that it is quite common. Next time when your child wants to go out and play you will never leave her to play outside by herself and you will try to keep her always in sight.
Get to know about the bias. Learn about the fallacies and cognitive limitations. Acknowledge the limitations. When you design a product, instead of just testing within your team, test it outside the team. Invite other team members to join. Don't just stop with that, know your users, bring in more people from outside the company to test your interface. Notice how they feel when they interact with your product, get to know their emotions.
Always remember that the user doesn't necessarily have to be a developer, engineer or an UX expert. And here's the fact, if an user is unable to figure out the interface. It is the developer or the UX designer is stupid, not the user.
Accept your vulnerabilities and try to overcome it.
An user interface is like a joke. If you have to explain it, it is not that good.