I guess what happens here is that 1Password actually has two different versions: one distributed from AgileBits and one offered on the Mac App Store - and that the two versions follow different versioning patterns. The AppStore 3.9.6 version came out roughly at the same time as the regular 3.8.19, which probably means they're virtually identical. This implies that the version you've got now, 3.8.20, is actually newer than 3.9.6.
Sadly, Mac Informer can't differentiate between these two patterns and simply notifies you of the highest available version number. Maybe it'd make sense for Mac Informer to follow AgileBits's pattern, leaving the AppStore version updates to the AppStore itself, but some might take issue with that as well.