There are two reasons for this. Firstly, we presume that the user has no musical knowledge, so metronome beats per minute (bpm) settings probably mean nothing (although the HT-300's play screen does show the current bpm). Changing the speed by 10 or 20% is likely to mean more to most people.
Secondly, the HT-300 plays standard midi files via its compact flash memory card. A midi file may contain speed changes at various points, and these changes are stored as absolute rather than relative values (rather daft, but there it is). Consequently a speed change to 90bpm will always be just that when the midi file is played on an ordinary midi file player, regardless of the initial speed selected for the tune. In reality, if the tune is played (for instance) 15% faster, then any speed changes should be affected similarly. The HT-300 treats all speed changes as relative, thereby producing a more musical result.