Oh! Ok, cool.
I knew the groove settings did not alter the bpm of the song, but until now I always just assumed it was the same (or very similar) as manually placing an 0Dxx on every other note in every track.
For example, when using speed 06, I thought a groove of 50% simply equated to 0D03 / a delay of 3 ticks. Giving the same results as if I had done:
00 | C-4 … 0000 | — … 0000 | D-4 … 0000
01 | C-4 … 0D03 | F-5 … 0D03 | D-4 … 0D03
02 | C-4 … 0000 | — … 0000 | — … 0000
03 | C-4 … 0D03 | F-5 … 0D03 | D-4 … 0D03
etc.
Thanks for clearing up the mystery 