If I understand your question correctly, I believe my Boxster does the same thing.
I can fill up on level ground, and I let the gas pump stop when it detects the tank is full. I know it is full, because if I try to pump any more gas, the tank overflows (done that twice, but no more!).
The gas gauge not only doesn't show completely full (it's just barely below the full mark), it also drops very rapidly. Within 90-100 miles, the gauge is down to just barely above 1/2 tank.
The last half of the tank goes much slower. It will last over 200 miles.
Bottom line is that the gas gauge is not accurate or linear. It could be the sender unit in the tank, or the gauge itself, but since I know how it acts, and I always reset my trip odometer when I refill, it's no big deal.
Jack
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