He joined up with the Kings Squiggs. It was later in the war so he was sent where he was needed, so transfered to the Welsh.
I dont know too much about this subject but he probably enlisted and trained with the Kings, and the regiment he signed for was overseas. So the Army sends him where he is needed. The Welsh where most likely getting ready to go to France and he is sent there to boost numbers.
It happened to a lot of men. Some 3 or 4 transfers.