change WS fluid = increased mpg?

I have a 2nd generation Prius w/ 65,000 miles which I bought used last winter. It was getting around 53 mpg in mixed freeway and around-town driving, which I thought was pretty good. Recently I took it to the dealer for routine maintenance. I had them do 4 things: replace transaxle WS fluid, inverter and ICE coolant replacement, and brake fluid replacement.

After this change I notice I get significantly better gas milage, now about 57 + mpg. As far as I can see everything else has stayed the same, same driving habits, same tire pressure, etc. Now I'm not complaining, but I'm just surprised that (presumably) a WS fluid change would result in an extra 4 mpg. Is there something here that I don't understand?

 

I agree w/ Patrick that it's unlikely that the transaxle fluid change affected the mpg. My best guess is that something incidental to the dealer work resulted in better gas mileage. Maybe they disconnected/reconnected the high voltage cables as a safety measure which resulted in a tighter electrical connection? I can say, judging by the amount of dust on the engine, that other than for oil changes nobody had touched anything under the hood since the car was new.