Do you have some digital calipers? Can you see if they are cut to the size you expected them to be?
And I second what fish tacos said, you can add a variable specifically for your tenon tolerance, and then add it to each dimension related to your tenons and mortises, and then you can adjust the tolerance by changing that variable. But if the error is that your bit is smaller than the CAM software thinks it is, then a tolerance is just going to cause you headaches in the future.
Also check that you are cutting square, you should be able to see where it is wearing/rubbing and that might point you to some issues with the bit not being perpendicular to the surface, or the XY isn’t square. Both of those could cause tigher joints than you expect.
And just to check, you are overcutting inside corners, or undercutting outside ones, right? So that you aren’t fitting a square peg in a 1/8″ rounded rectangle hole?