Clearly Roger is a superior offensive player. There are a lot of factors involved though, such as his far superior serve, which gives him more aces and more easy put aways. His serve alone is good for 15 winners in a two set match, between aces and 1-2 punches. Not that his forehand isn't offensively superior, because it is, but your take on winners count has always been somewhat narrow-minded.
It's not a coincidence Nadal's winner count catapults significantly when he's serving well (see 2010 and 2013, when he "out-winnered" pretty much everyone). Conversely, when Rafa's serving like a WTA player, he won't produce much winners. It's obviously not the only reason but it's a huge factor. Look at how many floaty weak returns Rafa produces, and how many easy put aways it gives his opponents. That's obviously a knock against him since his return is iffy, but it also adds to discrepancy in winners count.