NOTE, Apple released TWO different 10.14.6 Supplemental Updates, one on Aug 1, and another on Aug 26, 2019. To confuse matters, both share the exact same name, and the download link is the same (basically they slipped streamed the second update in). Update #1 will update your system to Build 18G87, update #2 to Build 18G95.
If you're wondering why your Mac is prompting you again to install the Supplemental update, this is why. If you installed the Supplemental PRIOR to Aug 26, but aren't getting prompted to install again (i.e. #2), you may want to get #2 and install anyways, as it sounds like there was a bug regression or incomplete fix in the #1.