Tape is rarely an exact length, though always more. And generally about 3-5 minutes worth.
VHS-C was TC20 or TC30 speeds.
To get 1 hour, it must mean that the TC20 was EP/SLP mode (3x).
So if the tape had a 5-minute overhead at TC20, then it would have 15 minutes at EP.
Thus 1:15 EP mode VHS-C tapes.
Most people actually don't know this, as most homemade tapes are never filled entirely.
But the camera/VCR will record until the tape runs out, not a preset times (unless scheduled/timered).