Help explain daily variation in indicated MPG

Here is the thing. Sometimes, out of the blue, the indicated MPG for my commuting trip shows too high number, something like mid 60's. Sure enough, next day, I can count for the indicated MPG to go under the average, something like low to mid 50's. The average is high 50's (translated to mid 50's MPG by the gas pump). The difference cannot be explained by ambient temp (not much variation and garaged car) or battery state (I have constant charge with my style of driving) or difference in the traffic (very predictable). I'm puzzled about it. What is striking is the compensation next day after.

 

11 miles.

 

No, it's not that. I explained it in my OP.
The more I'm thinking, maybe I'm trying to see a pattern in random events?

 

Speaking about rain, when it rained hard (flooded streets) couple of days ago. The average round trip MPG plummeted from near 60 to low 50's MPG, meaning the return trip was close to 40.