Why does my shutter speed vary between Program and AV mode with the same Aperture?

I may have overlooked something here. When I am trying to shoot low light with my SLR with a flash, I get a much slower shutter speed when using AV mode instead of P mode. I understand ISO, Aperture, and Shutter speed make up exposure. In both instances, the camera was set to ISO 1600 and the Aperture Value was at F/2.8. The same shot was used. P mode gave me a shutter of 1/60 and AV 1/5. What am I missing here as to the effect of the flash in AV mode versus Program?