Look at it this way.
Your main function is something that totally defines you.
You think that everyone can do it just like you can, but that is just a side efect of watching from subjective position.
For example I could not understand what Ni is. Descriptions did ring some bells but I could not see it. Plus Ni is probably the most illusive and abstract function of them all.
On the other hand I have very well developed Te and I am quite J. Te is hard to miss, actually it is impossible to miss it.
But with time I have come to understanding that the "bastard "is invisible almost all the time and it is invisible since it is introverted function and it is not all about what ideas/information will I get. It also about the way my thoughts will be created in the first place. So the "show" goes since I was a baby and it was hard to pinpoint something that is always there. I mean always as always.
With time I have learned that I can do things with my mind that are out of reach for other people (at least this is how it looks like)
Maybe I am wrong but this is how I see it and I presume that similar story is with Si. Probably it is easier to detect it since it is more earthly, but I think that trick is same.