I get this frequently too. My ENFJ and ENFP friends think I'm a very caring person and that I go out of my way to help others. My ISTJ dad thinks I'm too nice to others. And I don't quite understand why. The only thing I really try to do is to be a decent person.
What I think happens with INTJs, especially those that appear caring to most others, is that they were taught some good social rules early on, that these were "this is how you deal with people" rules. For TJ types, these rules are regarded as pretty hard and fast, you don't change them, because if you start trying to guess them or figure them out from scratch TJ (lacking Fe) gets it wrong.
So as long as we were reasonably socialized as kids, we tend to treat even complete assholes "decently", without blowing up, without apparently acting immature, using simple rules of "being nice", and calmly, intellectually approaching problems from an objective standpoint. Why? Because those are the rules. The FJs, especially if they can read us, note that while we might be clumsy, that there is very little in the way of meanness or spitefulness. The FPs note that we're being intellectually objective in exactly the way they
want to be, even though they find it difficult to be that objective.
What's funny is that they're all reacting
warmly to me, when I'm being, for the most part, logical and reasonable and problem-solving, which involves very little emoting (there's the occasional "wow, this is a really cool idea" kind of emoting, but I'm not emoting to express my own warmth or to make them feel warm).
Mostly, though, I think they start noticing a certain level of patience, of taking people at their word, of dealing forthrightly with matters, and to a lot of people who are used to others being snarky, spiteful, backstabbing, grouchy, etc., this near-emotionlessness of the INTJ appears ... well ... "nice."
This isn't the case for all INTJs. INTJs who are too wounded, or self-involved, or possess any number of other of issues of immaturity, won't appear this "nice" and will often have a rather harsh "bite" to their self expression. But for most reasonably well-adjusted INTJs, yeah, they'll come off as very "nice."