XC70: General Fair bit puzzled over mpg..

[ThomasG]

So.. long story short.
Came back from a two week trip to Poland.

Car is 2005 XC70, 2.5t, often producing slurping noises in the fuel tank (by the book, it's supposed to average ~25 mpg)

Now.. on the way "out" , to Poland, I didn't pay much attention to fuel consumption, other than noticing that after filling up at shell bead Dusseldorf, (e95, cheap one), tank was enough to cross the Germany and get to petrol station some 30 miles into Poland, with 15 litres still remaining in tank. That's roughly 750km/some 450-500 miles.
Fill up to brim at Orlen, again cheap e95 and .
70ltr tank wasn't enough to make it to Warsaw, 400km/250 miles away.
Strange, but.. that doesn't surprise me as much as what follows:

Return trip, I've been using e98, again from Orlen, for first 300miles, and e95 from shell for remaining 1000 miles (yes, 1300 each way).

On the piece from Warsaw to same petrol station by German border I used 30 litres..
And average for whole return leg... 32.5 mpg.

I spent whole morning yesterday looking for that missing fuel stop, it's just impossible that this car, with this engine, loaded to its absolute limit, used only 180 litres on 1300 mile leap.
Impossible.

Even onboard computer says AVG 26mpg.

But I checked. That's all the fuel I put in.

How. The. Heck. Is. This. Possible ?