From the official KoT facebook. Source
We're going to announce some changes that came into play lately and affected matchmaking. They improved game experience of vast majority of players remained unnoticed by them because of this. However veteran thieves feel that something works different.
First of all, recently we've changed matchmaking groups a bit. Every level now has it's own group. It means that if you're level 5 thief, you'll find your victim among level 5 thieves first. However several things prevent players from joining matchmaking. If they are online, under attack or they've been robbed not so long ago you can't find them. In this case matchmaking will search a rival for you among level 4 thieves, then among level 6, then level 3, then level 7 etc.
There are two big exceptions here: level 45-50 and level 51+ groups of players. Players in each of these two groups are treated as like they've got the same level. So if you're level 100 and can't find an opponent in your group, matchmaking will try to find it in neighbouring 45-50 group, then it will search in level 44 group etc.
This also means that level 43 and 44 group members can sometimes find someone in 51+ group (which is also called 'hell' because of dungeon design difficulty level and stealing skills of thieves there) for attack and get attacked by thieves from this group.
Have fun, thieves! Enjoy your groups and hell too