What frame rate is used by iPhone in "30fps" mode?
30 frames per second is based on NTSC broadcasting standards that go all the way back to the invention of television.  All recording equipment conforms to this spec even though we don't use analog anymore.

NTSC actually runs at 29.975 frames per second, so both your apps are technically correct.  We just say "30fps" because it's easier to say and write than 29.975.