Simplest reason I see for increased speed when you don't have a huge edge is something like the daily Tier Credit bonus at a Harrah's property.
If you're trying to hit the $10k coin in threshold for a day and like playing single line quarters, it's a huge difference in number of hours grinding between 800hph and 1200hph. One is a long day of play, the other is miserable work.
The answer of "just play a higher denomination" doesn't work if you're a disciplined player who respects bankroll requirements.