Through diligence and with the help of VendenBos, I finally succeeded in figuring out the reason for the discrepancy in the number of records in OCD vs. the number of albums in my R&T collection. For reasons unknown to me, apparently the designers decided to assign a different protocol to OCD and R&T for counting discs/albums, etc.

As VendenBos posited, OCD treats each CD as one unit, where R&T treats each title/album as one unit, no matter how many discs may be included. It would seem more sensible to have both the software and online catalog match in every way, but ...

I discovered the confirmation of this last night when (for the very first time) I clicked on Database