I'm not sure I get the part about setting the flash ev to -1. According to the EXIF information on the picture, it was shot in Aperture Priority at f/4.0 at 1/4,000 sec. No flash was fired according to the EXIF. Granted, if it was a studio flash or some such, the EXIF wouldn't have shown it.
So, at 1/4,000 second you would have to set a Canon flash on High Speed Sync or the shutter will reveal only a tiny sliver of flash effect on the picture. Perhaps when the camera is in Aperture Priority, it knows this and won't fire the flash if it couldn't possibly work. I don't know.
Also, when shooting Aperture Priority or Shutter Priority (Tv) with the flash set to ETTL, the camera and flash assume you are using the flash for fill and will adjust accordingly.