I'm pretty relaxed, almost dead, the majority of the time. However, if someone acts or says something completely out of order, (according to my values) then my inner spark will ignite... Not particularly aggressively, (because I'm a mellow yellow) but I will make my opinion clear nonetheless. Also, I find no trouble in debating with people in order to convert them to the right belief (my one ;) )
So, to answer your question, maybe because INFP's have a more strongly felt value system. Also, I feel that INFP's care less about what people think about them and strive to just stay true to themselves, which could lead to more arguments.. Though overall, you will find both INFJ's and INFP's to be rarely confrontational and reasonably calm and collected characters!