2.1.424 Part 4 Section 22.214.171.124, rsid (Single Session Revision Save ID)
a. The standard states that the rsid should be randomly chosen and that every editing session shall be assigned an rsid that is larger than all earlier ones in the same file. If both are observed, the application would quickly run out of allowable rsid's.
Word generates a random number for every rsid that is not necessarily larger than all earlier ones in the same file.