In the unit_test.php file, just add:
echo FinanceData::GetFuelPrice(1000, 'LFPG');
I think you're using LFPG. See what that says.
OH! I just realized why - it adds a 'surcharge' to the fuel price (for taxes, costs). This is the FUEL_SURCHARGE option (it's default is 5, which is 5%). You can set that to 0 if you'd like, but 5% is the default (some 'extra' realism)