Yep. The exact placement of the monopole w/r to the close can wall will change the the polarization which changes how will it will get through the imperfections in the horizontal seam slots which will be polarized. And that's ignoring the that different places will be better or worse (vs frequency) for exciting the can's cavity modes (at $frequency) in the first place.

But a long distance high power RF source can have these properties altered too by the type of emission mechanisn, reflections, scattering, etc and items placed in such a faraday cage will be located at good and bad places to excite the cavity mode too.

So it's kind of a wash and probably okay for this kind of test. The fact that the electric and magnetic components of the field aren't in balance (near field) doesn't matter too much.