The most likely answer may be that the 18238 watch and the diamond bezel are both genuine factory Rolex, but they were not “born together.” (You are correct that, if they’d been born together, the case’s reference number would be 18348.)
Rolex used to encourage its ADs to upsell diamond bezels separately, to add some new appeal to existing watches. This bezel could have been added by the AD on day 1 when the watch was first sold, or years later.
This thread may offer you some more answers -
https://www.rolexforums.com/showthread.php?t=289046