The image above is taken from the Cart Bot svg file. The difference is 0.6mm, which is big enough to prevent the pieces from fitting snugly. So the tab is actually 9.7mm (0.15mm taken from both sides of the width = 0.3mm lost) and the slot is 10.3mm wide. Why? Because the kerf of the laser cutter will remove an extra 0.15mm from all sides. If you design the tab and slot to both be 10mm, then they'll fit loosely and fall apart. Let's say you want a 10mm tab to fit snugly into a 10mm slot.
To get pieces to fit snugly, you need to design the connections so that one part is slightly larger than the other while also accounting for the width that will be lost in the kerf.
