A normal double of 52-42, for instance, requires a greater difference between each cog in the rear cluster, because you need a significantly larger cog for your low gear. My 36 tooth small ring needs only a 23-tooth cog to give me the same low gear as my husband has on his 42 tooth small ring and 26-tooth cog. So given that you need a larger cog for your low gear, you're going to have bigger steps down the cluster as you shift to higher gears. The percentage difference between a 52 and a 42 ring is much less than a 50-36, so although you can get all the same gear-inch equivalents in some combination or other, they aren't all in a row when you're on one ring, and you have to shift the front more. I don't know if I'm making any sense here. I know what I mean, but it's hard to explain. You can explore and compare all the possibilities on Sheldon Brown's miraculously wonderful gear calculator, which you'll find here: http://sheldonbrown.com/gears/