Example: If an Apple Loop is transposed to sound 7 semitones higher, it will sound 5 semitones lower instead. So the Loop is harmonically correctly transposed, but not to the intended octave.
Transposing audio material is a technically complicated process which always implies a certain loss in quality. The higher the amount of transposition applied, the higher the quality loss. This is why Apple Loops are always transposed to the smallest possible value.
Keep in mind that the sound quality depends on the actual transposition based on the Apple Loop's original key – not based on the song key which defines the zero line of the Transposition track. For example, if the song key is already 5 semitones above the original key of an Apple Loop, setting the transposition value to +2 will transpose the Apple Loop 10 semitones down, because this is only 5 semitones below its original key (instead of 7 semitones above it).
In the classical European music system, an octave is divided into 12 semitones. As +7 semitones is therefore harmonically equal to -5 semitones, -5 is chosen as the transposition amount in this case. The same happens with other settings: A transposition of -9 will result in +3, and +12 will result in +/- 0.