He says upgrade and you downgrade, weird.
And if there is no one fighting at one's lv range, than that player's search expands and that player becomes able to fight against a higher lv or a lower lv player. This doesn't depend on your search levels.
A lv 20 player starts searching.
It starts searching at lv 20, finds no one at that lv, than expands to lv 19-21, than 18-22, and a while later, 15-25 if he doesn't find any opponents.
And at that moment (or a bit before, you might not get matched to any opponents as well during that), you start searching at lv 15. That you get matched with him because he's searching for lv 15 opponents, as well as all other levels between 15 and 25. The search brings one result, YOU, because there are no other people searching between lv 15 and 25 at that moment. And that's how you get matched to him.
To avoid this, try to login and search for a battle when many people are online on supermechs and searching for a battle. This will decrease the chances of this happening to you.I can't guarantee that it will not happen anymore if you do this, but the chances of this happening should be significantly lower.