Si creates archetypes from personal experience and compares what it faces in the present to that archetype. This archetype is solid, unchanging, completely static; if archetypes could change or be altered and re-defined, then there is literally no point to there being an archetype. This is why Si doms may have a reputation for being stubborn or "stuck in their ways" depending on the archetype.
For example:
One Si dom's archetype of a chair is a seat that has legs and a back. Imagine this Si dom sees a barstool and someone calls it a chair. They argue with that perosn. No, it's NOT a chair. It doesn't have a back and all chairs have seats, legs, and backs. This item does not qualify as a chair because it does not meet a chair's archetypal qualities.
BUT, unlike Ni (which I think it more about universal archetypes), Si's archetypes, being based completely on past experience, are unique to the individual. One Si type's archetype may not match another's.
Let's imagine the person the Si dom from the argument above was arguing with--the one who called a barstool a chair. Maybe this person is also an Si dom, but their archetype of a chair looks different; to them, a chair need only have a seat and legs. Thus, a barstool qualifies as a chair to them, but not to the first Si dom. And neither of them are going to change their minds about who's right, ever. Because archetypes do not, cannot change, or else they lose all meaning.