From the manual:
Sample Rounding and Edit Operations
When audio material in Pro Tools is samplebased,
some amount of sample-rounding may
occur with some edits when the Main Time
Scale is set to Bars:Beats. This is most evident
when you need audio regions to fall cleanly on
the beat (as when looping) and notice that the
material is sometimes a tick or two off. With a
few simple precautions, this can be avoided.
To ensure that selections are precise aligned to
the grid for looping, copying, duplicating, or repeating,
consider the following:
If you use the Selector tool, enable Grid mode
and select the appropriate grid value (such as
sixteenth notes).
For the greatest accuracy, type the selection
Start and End points in the Event Edit area.
Do not select the material with any of the
Grabber tools (or by double-clicking with the Selector
tool).