Thanks for you answers it helped me out a lot ! I didn't know about the [i,j] notation but thanks to you guys I do now !

Well in fact CodeTools:-Usage only gives you "memory used", "alloc change", "cpu time", "real time" within a Maple session. But the computer is doing some other stuff in the background while it is computing inside Maple, then the time it takes to compute will change a little bit according to background tasks.

I was referring to AIDA32, SANDRA, EVEREST for example.