I think the problem is that the people entering the data (squaddies) were not very motivated so there's the least possible info to the lowest possible accuracy. I paid the £35 and got a copy of my B card form REME, but I'm not sure "being able to write" was a required skill at the time it was filled in.

If you know the military ERM a lot of the MERLIN data is now here.

https://www.merlinarchive.uk/

if you know history of a particular ERM and it’s not on there. You can add it.