He does actually do that though and I think thats an important distinction because the cars own reporting can be bullshit.
I do and it is. Apart from that this is a very flattering thread.
Reasons I've found for reduced MPG include
When it's colder like it has been of late engines take a lot longer to warm up. Any journey of under about 20 miles will show this up. The longer the journey the less the effect because you spend less percentage of time running cold.
A change in route or use
This is the biggie as it effects everything. I've got a graph of something like 40,000 miles stored for out car and you can see average long term fuel consumption rise and fall over a period of months and I can tell you exactly why it happened.
Changes in the car
Not as in a different car but stuff like servicing, tyres etc. I noticed a slight improvement after we had the first new set of spark plugs. The engine was noticeably smoother and more responsive and this seemed to translate into better MPG. Eco tyres didn't seem to make the slightest jot of difference but pressures do, slightly. Retro fitting cruise control made ours worse. That, I think, is partly because its not as sensitive as an OE system and partly because I'm one flippin' light footed bar steward and I can beat the cruise at its own game.
Personally, for the OP, I'd say there's a slight chance it's a mechanical issue but more like the fact that this happens all the time, but not being a geek (hello!) you've mover noticed it before.
I'd be tempted to say its the weather to be honest.