GPA are using the same signal I believe the 2600 rounds slightly different - you have probably seen that on your documentation- often times a rate entered on the brown display as a round number shows up a hundredth or two off- nothing to worry about.  Acres are never going to agree as they are calculated differently- the most accurate source for your acres if you are looking for a tally is the brown box.  If I am in a field that I have never sprayed before with unknown acres I snap a boundary and then view total acres in my mapping key/boundary tab.