MIdi Transpose in Diatonic mode a bit funky

I was playing with brass voicing and stacking down from the top note with the MFX Midi Transpose and things didn't go as expected. It seems negative scale steps are a bit of a mess.
 
(Note: as the scale steps of 0 = a first, then 1 = a second, 2= a third, 3 = furth etc , so an ctave is +7 not +8)
 
If I go down -1 scale step, instead of going down a second, it goes up a seventh. So plus an octave then down the -1 scale step.

 
This continues with each step, -2 is up a sixth, -3 up a fifth all the way to -6 up a second.
 
But when it reaches -7 it jumps down a octave to where it should go.
 

 
After minus 7 it becomes predictable and seems to be an octave out from what it should be. Minus 8 acting like minus 1, and minus 9 acting like minus 2 etc. Until minu 20 then it jumps an octave down. 
 

 
 
12 up13th11 up12th10 up11th9 up10th8 up9th7 up8th6 up7th5 up6th4 up5th3 up4th2 up3rd1 up2nd0 1st-1 up7th-2 up6th-3 up5th-4 up4th-5 up3rd-6 up2nd-7dwn8th-8dwn2nd-9dwn3rd-10dwn4th

Thanks.
 
It's a pretty cool tool. I think I have generally used it building from the root up. and adding an octave below, so hadn't picked up n this before.
 
The key scale optoin is pretty cool for checking out your midi in different modes. I use a plug in for paying chords with one note. Once I have mapped out voicings in C major, I don't need to repeat for the other modes via this MFX