In Products module, the Modified date and Modified by fields at the bottom of the Main tab only counts the changes made on the Main tab. So, they don’t correctly reflect any of the changes made on the other tabs, such as the Properties or Documents tabs.
This is misleading because there are no individual display of Created/Modified on the other tabs. Intuitively, Modified date/by is understood that it counts all kind of changes made to the product; there’s no distinction between the tabs.
The suggestions are having a separate Created/Modified section on each tab, or counting all changes towards the Main tab’s Modified fields.
This is also true for Orders module which also uses tabs.