I must have misunderstood the question, I guess Weaver was just asking the differences between the A and B types. I was under the impression that type X had been seen somewhere.
Not just you, I thought that was what he was asking too.
But simply put, all router manufacturers have a habit of changing the internal hardware over time (or even regions) without renaming the model, sometimes it will be completely different hardware. It does seem dumb, but basically in their mind the models are "comparable" even when in reality they can even have more or less RAM, slower/faster SoC, smaller/larger NAND, so really should be different models in their own right.
BT just happen to name it this way when its what they consider to be feature "comparable", rather than increasing the number, but to show they are different internally when the outer casing looks identical.
The most obvious reason why they need to do so is of course the PSU connectors and ratings are often different (the former likely to make your average person double check as if they had both they would assume they are interchangeable, as people do).
Of course other manufacturers like Zyxel have that letter on the end and are more prone to changing the case too so you can see at a glance its different.
There is also the fact any changes to the hardware have to pass the FCC tests, AFAIK you can't change anything beyond maybe component brands without needing it testing to meet regulatory requirements again.
