The filter for mythic+ key level does not work anymore.
Also, the overview shows differing numbers for the mythic level than what seems to be advertised. The top +14 group shows as +13 etc. So either everyone is misadvertising it or the addon does not read something correctly.

The "M+ Rating" filter should filter by rating, not by key level. According to your screenshot, you should only see groups where the leader has a rating of at most 13.

Your screenshot still shows groups with a rating higher than 13, however all those groups are green colored, meaning they're new groups. For those new groups, the information about the leader and her rating is delayed and filtering has already been done when the rating arrives. This is usually fixed with a second click on search.

Filtering by key level is not possible for addons due to restrictions from Blizzard. You may however type something like 14-16 or 15-15 into the standard search box.

The number underneath the rating in the list is the highest key level the leader completed on this dungeon. It's gray if not in time and white if in time.

Please also have a look at the FAQ in the Wiki tab. Almost everything should already been in there.