I think it's because it's designed for retrofit: if the original was a single 13A socket, the new attachment is also limited to 13A. That way a householder can do a replacement wthout need to verify current capacity of the circuit (eg if this was fitted to an existing spur which the rules say you can't add extra sockets to).
Fitted one of them and I like it - it's not deep unlike others, so it's barely thicker than a conventional socket. There is a green LED for the surge protection which might be annoying in some cases (or tape over it).
I fitted a 1 to 2 by Eterna. Not here and house was sold a year or so later so I don't know how it lasted. Seemed OK build wise but even to my catholic taste (or lack of taste) looked clunky.
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