The obvious explanation is that it's been exposed throught the backing paper, by loading or unloading it in bright light.
However I've had the same effect from older film, and I suspect that the ink can react with the emulsion over long periods of time.
If it were exposure through the paper then each frame would have it's OWN numbers on it (taking into account the three different numberings), while if it's an ink reaction then it would have the number of a frame before or afterwards, depending on if the reaction was before or after exposure.
Ian