Yea Introverted Sensation is just subjectivity of sense perception. Favoring what you get out of an experience rather than the actual components of the experience itself. So because this is subjective, it means that an experience might bring back a memory or something from within, but not necessarily. It could just be as simple as only focusing on one aspect of a sensory experience.
For example I have heard many Si-doms describe places by temperature i.e. Chicago is cold and Miami is hot or Seattle is grey. Now a Se-type might do this too, but I think these associations have more personal significance for the Si-type (like I went to Chicago and it was cold, so now when anyone says Chicago I think 'cold.') Objectively Chicago isn't always cold, in fact its often hot and muggy in the summertime, but the Si-type would, to me, have a tendency to take their own perception more strongly and downplay the objective criteria and therefore Chicago becomes 'cold.' Or Seattle becomes grey (which is really a subjective way of looking at it anyway, because grey means something different to each person) when in reality Seattle is a very beautiful city with lots of evergreen and deep blue water, white puffy clouds, etc., but this subjective idea of 'grey' becomes what sticks in the person's mind and they downplay all contrary external data. This is why Si needs Ne, to help remind them that things may not always be what you perceive them to be. To give them a sense of possibilities beyond the limited scope of their perceptions.
My examples about cities are probably not good ones, but definitely it becomes very clear when you look at art or photography. Oftentimes a Se-type might look at a picture from a Si-type and wonder "why did you take that picture?" because the image communicated something to the Si-type that the Se-type, choosing not to project anything from within but take his sensual perceptions at face value, doesn't see.
I showed this picture a while back but when I first saw it, my first impression (as a Se-type was) wow this is a messy, barren room with a weird pillow. Why would anyone take this picture, much less put it up on a website?
Until I found out the photographer, who I know to be a Si-type (INFP I believe) was taking pictures of spaces where people had just died. The emptiness of the space represented something that only she understood, and I can never know what she saw that made her take these pictures, or choose these particular images, or frame them the way she did, etc., (with Se its all a lot more academic - you can look at an Ansel Adams picture and technically understand what he was trying to do). She probably couldn't explain it either. It's just a way of looking at things via your five senses that is heavily influenced from what's within (of course that could be memories, or emotions, or whatever, but might also be something else - could be something really abstract, or even something archetypal like 'evil' or 'fear' or 'home').