Which exposure mode are you using? P or AV/Tv/M? Do note that in the exposure metering for flash usage works different for both. This is not flash exposure, but exposure for ambient light. In P, the flash is considered main light. In Av/Tv/M the flash is considered fill light and the ambient light is considered as main light. Metering will read the ambient light first, then fill the foreground / subject with flash.
If you fix the shutter at 1/200 and set a certain aperture (which one, actually?), the camera has only left the Auto ISO to adjust according exposure metering result.
More to read: http://www.photonotes.org/articles/eos-flash/