Primary Stroke Change?!

I grew up fearing Butterfly like so many other kids.  Lately, I'm more confident in my butterfly than any other stroke including backstroke.  I've learned it's good to switch up your training every so often.  It seems like every set lately has been a struggle to swim backstroke.  I'll usually start the set backstroke since it's still my primary focus, race hard, only to get mediocre times from the coaches.  I get a bit discouraged, and switch to butterfly for the heck of it.  The set immediately gets better!  I feel strong through the whole race!  My times are proportionally better across the board.  Somehow I'm becoming a butterfly swimmer, something I never really wanted to be.  Today we ended our race pace set with a 50m off the blocks for time.  This was at the end of 3 other 50's and I had about 30 seconds rest before the gun went off.  I touched in a 25.5, a very respectable time for practice!  Comparatively my 50 backstroke with the same rest earlier in the set was a 28.8.  That's a significant difference even taking into consideration the start difference which I've learned is about 0.60 seconds.

I'm making progress, just in different areas than I expected lately.