The BX is a suspended deck, as is the B2601.
I haven't had a BX for a while, but the areas to look at are:
- The 3ph actually lifts it, so if the 3ph arms are raising unevenly then that could cause what you note. That can be simply adjustment on the arms (if they're out of level), or could be something broken. If I recall correctly the single lift mechanism lifts both, so it's not like there are two separate rams and one could be leaking - so it'd have to be something bent or the 3ph arms adjusted so one is lower.
- The mower arms themselves can be bent where they connect to the 3ph mechanism. This could result in one side lifting sooner than the other, because it's bent in some way
From your pictures, it doesn't look particularly uneven. It might be the angle, but it looks like the gap between the frame and the lift arms is about the same. But one is a bit further away - so are they actually the same, or is one a half inch and the other an inch gap?
If the level-ness of it changes as you lift, it sort of suggests that something is bent somewhere, causing it to have a smaller arc of travel or something. Does it smoothly change level, or does it come most of the way up evenly then one side stops and the other keeps moving? If the former then I'd suspect bent, if the latter then I'd suspect the 3ph arms are out of adjustment, so one reaches the top of it's range of motion before the other.