I think its an impossible question to answer without speaking to the people who do this, and even then they may not truly understand why they do it themselves. I'm guessing that if someone is already at the point of self harming they are already very distressed/upset/ in pain, and they post online in order to seek some form of solidarity amongst others in a similar situation. As for captions which sort of play down the exent of their self harm, may be its a way of expressing some form of self depreciation (if that's the right term) or belittling their problems, as some , though not all, those who self harm often have very low self esteem and see themselves as worthless at least some of the time, and being self critical and both down playing their self harm is one way to express this.