Aussie David;
It's been estimated that Winter temperatures decrease mpg by 10%. Of course, 45-50 degrees isn't bad compared to what I see here in New York State during the winter. It all comes down to how long the engine takes to get up to temperature and for the IMA to begin functioning.
Also, alot of the folks here will seek out the very same pump everytime they fill up so that the calculations will be more uniform. It eliminates one more variable from their figures. It certainly can make a difference as you've shown.

Scott