I think this is because MSL was originally scheduled for the previous window in the Earth-Mars transit (about 2 years prior), but was pushed back.

Correct. It was scheduled for 2009 but due to problems with the new dry lubricated motors and software issues they decided to delay to the next launch window which of course was Aug 2012.

A very nice documentary is currently running on Discovery which goes into much detail on the construction of MSL.