Why would a manufacturer change chainring sizes drastically between years?
Well, in 2010 there's the S1, S2 and S3.  S1 is lower-end and has 50/34.

In 2011, they dropped the S1 and there's only the S2 and S3.  (based on what's up on their website right now)

The lowest end of the line is probably considered something like "entry level" or at least "not for racers", so it gets the "slower" chainring setup.  They've kept the chainring size on the cheapest bikes in the line the same, they just moved the labels for what the cheapest bike is.

And it's probably all about marketing and whatnot, nothing to do with any real solid technical reasons.