Like you said there seems to be a little bit of a delay between posting an update in the update log and the submissions being bumped to updated tab. The same behavior is true for the project section of the site, but interestingly for both is that the "last updated" attribute of the submission shows the time when the update is posted, not when the submission actually gets bumped.

In the future you might want to open a Ticket as the sites developers do not actively read the forums, but do check tickets, you can try asking why the delay exists if you are still interested in finding out but the response might be a little delayed