I must admit that I haven't been using slide film on a regular basis for two years or so, but the first thing to do in an odd result like this would be to directly compare the meters in the cameras - probably with artificial light indoors so that it doesn't change during the metering session.
Of course there is no need to make any exposures when comparing the metering. If each camera meters precisely the same (unlikely!), or you have been using the same handheld meter for all the shots, then make/buy/borrow a shutter-speed tester and compare the indicated speeds with what you actually measure. On electronically timed shutters they should be pretty accurate and repeatable.
Other variables may be the processing the films have received (accidental small pull and/or push at different labs, or between different home-processed batches), or some metering being done with older light sensors that are more affected by colour-temperature of the light, or even a DX-sensor fault, or . . . ?? In any case, if the difference is consistent, then it would be possible to 'simply' make a compensation when you use the affected cameras.