You've conflated TOPS-10 and TOPS-20. TENEX was developed externally by BBN, took advantage of BBN-built add-on paging hardware, and was quite different in both look and feel and its internals from TOPS-10, which had originated on the PDP-6 and was simply called "Monitor" at first. The name TEN-EX referred to its ability to run TOPS-10 executables in compatibility mode, as BSD variants could once run Linux executables. When TENEX was brought in-house by DEC, it was renamed "TOPS-20", since the Monitor had already been renamed "TOPS-10". The only difference between the PDP-10 (or DECsystem-10) and the DECsystem-20 computers was the color scheme and the OS supplied with it.