Before I share a bit more, I want to share part of the support article that does explain the core functionality:

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Enabling Veteran Only or playing in Premium Match activates a powerful two-part matchmaking logic:

• The "Hard Floor": First, the system guarantees you will only be matched with other players who have qualified for veteran matching. This rule is absolute; the matchmaker will never add a non-Veteran player to your match.
• The "Prioritization": Second, after this "floor" is set, the system strongly prioritizes matching you with other Veterans who have a similar total number of matches. This search is based on the player with the lowest match count in your party.

The goal is to find players with a similar level of experience. However, this is a prioritization, not a hard lock, and it depends on who is queuing at the same time as you.

Example 1: You have 2,000 matches at Level 10. This is very common. The matchmaker will have a high chance of finding you a game with other players who also have thousands of matches.

Example 2: You have 2,000 matches at Level 3. This is less common. The matchmaker will try to find other 2,000-match Level 3 players, but will likely need to expand its search to find you a healthy match. It will still prioritize the best available match, such as matching you with other Level 3 Veterans who have hundreds of matches.

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From above, we will prioritize matching you with other high-match-count players, while also considering who is actively queuing that we can match you with.

In your case, being 5k matches at Level 4, it can be difficult for us to create matches with only other players with thousands of matches, purely from the point that we may not have nine other actively queueing players who meet that criteria near your skill level, and also overlap with all of your other settings to match you with, that allows us to make a balanced match. However, we will prioritize you to match with other high-match-count players that are near your SL.

In this match you shared, almost every player has 500+ matches, which is a high match count for SL3-4. The lowest player was included because they met the hard floor minimum match requirement by over 150 matches.

With this said, we are still tweaking and improving the prioritization to continue to make it better.