The discussion about inconsistent back axle alignment on Fiats has made me notice that the car actually tracks better with its new axle than with the old one. So its likely one wheel (or both) was not properly aligned to the car body.
Somehow this doesn't surprise me; Panda 169's are notorious for having rear alignment issues.
Perhaps the 500 beam is made to tighter tolerances; if it weren't, we'd be seeing a shedload of posts in the 500 section regarding rear tyre wear, and we aren't.
FWIW, my 500 tracks much better at the rear than my Panda. That's only a single sample, but it concurs with what you have found.