Look at it this way and you can see it's consistent:
EDIT: Sorry, my original post's code was not actually very illustrative of the consistency - what's there now is better.
Anyway, to give an example: If you try 9/2 (which is 4.5) Blitz will return 4, which is rounded down. Well, if you round down -4.5 then you get -5. This accounts for the discrepancy you are seeing.
Open in online IDEa=-10 While Not KeyHit(1) Print a+"/2 "+a/2 a=a+1 WaitKey Wend
EDIT: Sorry, my original post's code was not actually very illustrative of the consistency - what's there now is better.
Anyway, to give an example: If you try 9/2 (which is 4.5) Blitz will return 4, which is rounded down. Well, if you round down -4.5 then you get -5. This accounts for the discrepancy you are seeing.