Similar UV shells look different after AO baking

Hello everyone! 

I am modelling a guitar in Maya. There are 6 strings of different width, but the meshes are similar, and the UV shells are similar as well. The UV-shells are located in the lower right corner of the UV map. No overlapping, no invereted faces/normals.

The issue: after the maps baking in Substance Painter the strings look quite different. On the screenshot below: two left ones are quite ok, I guess, but others seem to be of darker gray. 

I've been told that something is wrong with AO, but I have no idea what's actually wrong, since:

• the UV-shells are not overlapped,
• the meshes of strings have no N-gons or tris,
• there are no inverted faces/vertices.

So pretty much the meshes and shells are very similar, but for some reason the final result looks different.

I'd really appreciate your help! Thanks!