And it's not an adapter from RCA to XLR, it's RCA to 1/4" phono.
There is enough room in the center of the 3 pins for a female phono jack. It wouldn't be possible to have a female RCA there. So the adapter from the more common RCA to the less common phono is used. But an RCA male to phono male cable could be used in that case without the supplied adapter.
None of that matters to you, as this only applies to the newer subs, where the unbalanced (phono) and balance (XLR) inputs have been combined into one physical interface, but are still electrically separate.
The older sub, as you have, has the RCA and XLR both electrically and physically separate. I'm sure they were combined to save a little on parts and labor (plus the combined jacks are what the A1400-8/2 amps are using, so there's some standardization across the lines).
