at my homedepot there is a sign above the saw that says they will not guarantee exact cuts.
they also sell ind. pieces at my home depot from the factory. i have built racks using them as well and they are always the same size.
and i wouldnt recommend a rack with 3 tubs wide. it WILL sag in the middle. i built one with 2 tubs wide and it needed re-inforced with center dividers.
so they will only cut as close as the employee's are willing to try for.
adam jeffery