How accurate is a phone slow-motion function as a camera shutter tester?

I have been using my iPhone’s 240fps recording to test shutter speeds up to 1/250s, but I’ve noticed that all my measurements consistently show one stop of underexposure. For example, a 1/15s shutter speed appears as 1/30s in the recording.

I’ve tested this on three different cameras—a Yashica MG-1 and two Yashica FX-3—all of which I have serviced myself, and they all exhibit the same issue.

Could this be a coincidence, or is this method not reliable enough for accurate shutter speed measurement?