I bought the simple alignment part for the Combro (looks like an easy 3-D print job but hardly expensive). When used the combro is much more consistent and if not used being slightly offset may explain the differences between the 2 guns.
If anything the BFTA chromo gave my rig a slightly higher reading than on other chronos at previous events. As long as it passes I don't worry about the actual reading on the day. If the pellets match elevation clicks on the zero range a few fps won't make any difference.