I have a theory on this and it may well be wrong. I think it may be an apprentices test piece. Made oversize as they learn the process of manufacture. Other types of apprentices do similar things but in reverse, furniture makers make half size models to test them. However as a coin is small to begin with it would make sense for the apprentice to make it larger when learning.
The double stamping would not have stretched it, if it had Britannia would be double struck with one figure smaller than the other whearas the reverse is clearly rotated by a few degrees. There is more rotation the further away from the centre hence less error at the point where the trident touches the leg. Also both sides would have been effected if double stamping had made it bigger.
it is possibly thinner because they somehow used a standard size blank which would have had to have made slightly larger before being stamped.
just a theory imo