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TTL and Aperture vs Shutter Priority



I have been testing my Contax 645, Metz Flash and Contax 280 flash with some
B&W film to make sure they are in working order for an upcoming wedding. The
smaller Contax flash is for backup. I don't use the flashes often so am a little
confused by some of the testing I did. I tested both flashes in AV(aperture
priority) and TV (shutter priority)in TTL mode. In both instances the film was
exposed perfectly in AV, but was always overexposed in TV. In AV, I left the
shutter speed at a 60th and set the aperture at f8. In TV, I again set the shutter at
a 60th and let the camera decide what f stop to choose. Regardless of what lens I
used, it would always expose at f2.8 or f3.0 in TV. This doesn't make any sense.
Perhaps you are only supposed to shoot TTL in AV. Any thoughts on what might
be going on ? Thanks.