D800 - Is this normal?

I took several shots today with my D800. I accidentally left it on auto - iso..However I used flash since they were all indoor shots (inside a train). The flash did fire and all that, but when I checked the shots the ISO was like 6400 or something lower...Why did the camera still raised the ISO (basically still using auto iso) when the flash was used??? Is this a logical thing to do? I mean I like auto iso sometimes, but should not be used if the flash is on, even though one would select it...

Thoughts please?

Thanks,
vh