Here's a reasonably good explanation;

http://en.wikipedia.org/wiki/Extra_300

Be aware that all models, or at least the good flying ones, have some modifications from being true scale in order to make them fly better. That difference is best expressed by the examination of Reynolds numbers, but suffice to say that is the reason that you are allowed a 10% deviation from true scale on models used in IMAC scale aerobatic competitions.

http://en.wikipedia.org/wiki/Reynolds_number