This may be describing the [false consensus effect](https://en.wikipedia.org/wiki/False_consensus_effect), where people believe (incorrectly) that the majority of the population agrees with them about stuff.
The basis for it is that people will often use their own beliefs and behavior to model the behavior of others, basically as a primitive form of empathy.
The problem with this is that it assumes that other people have similar motivations and beliefs to you, which is often not the case, especially for people who disagree with you.
This has been widely experimentally observed, and is fundamentally caused by poor empathetic ability (a poor ability to understand how others feel) as well as motivated reasoning (people wanting to believe it is true). A lot of people exhibit this behavior.
Breaking yourself of this habit is necessary to see people as they truly are, but it is difficult for most people.
It links into a lot of other phenomena, like [attribution bias](https://en.wikipedia.org/wiki/Attribution_bias).
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