I see it more as ISFJ's seeking understanding, then being curious.
Having Si as a primary function, I usually don't need to talk to people around me about whats going on, because I have already inadvertently overhead. ISFJ's generally know a lot more then their exterior would give a way, and thus, people don't think they know that much. People who are curious often wonder How many tiles are on that ceiling? or I wonder how many times I said that word in our conversation where as an ISFJ would be like: There are 48 tiles on that ceiling or She said that word 10 times in a brief span of time in our conversation. I think if we lack any curiosity, it is because we already know the things that other people wonder about on a daily bases.
Things we are usually curious about are things we like(I am guessing that would be due to being a Feeler vs a Thinker). Having Fe, we are more likely to only such out things that would be okay to research as well. We are no less curious then any other type, our curiosity is just more focused, and so in a way, we can seem more curious. If another person spreads their curiosity out through many things, they don't get as deep into them usually. ISFJ's will generally hone their curiosity in on something they like and feel comfortable looking into, which can lead to much greater depths.
So, what could be define as "more curious" is up for debate. Are ISFJ's as broad spectrum in their curiosity, not so much, but is their curiosity abnormally deep, I would say so.
I have also read that ISFJ's often like "weird" things. I find this interesting. I seem to be a pretty clean cut guy, but I had many alternative friends and am into things like anime and manga.
