This is true in my experience as well. I write gen primarily, and I admit that I've been tempted to make my works sound, shall we say, racier than they are because I've found that a good number of slash readers dismiss gen and friendship stories out of hand. There have been some great discussions at metafandom about this phenomenon. I've never succumbed, but I've seen others do it and they tend to see a bump in readers and feedback. This ties fairly neatly back into the premise that we rate on the side of caution. We have few, if any, incentives to rate lower, but several to rate higher. Modifiers or warnings like slash and gen and BDSM and fluffy help inform the reader about how far the rating has been pushed towards the next bracket.