Why would one breaker trip more than another under the same load?
Get another 20A breaker and replace one of the 15A breakers with it.  There is a special exception in Code which allows 20A breakers to be used on  circuits where 


all the wiring is 12AWG Cu or larger
There is more than one 15A socket.  That's easy; most come with two. 


A 20A circuit can't feed only this.

  One of these is fine. 

FYI, there's also a UL rule that 15A receptacles must be good for 20A passthru. That applies to most GFCIs too. 



Now with two 20A breakers in the panel, you can experiment more and get back to us. 

Also make sure the breakers actually are the same make/model as the panel -- the simple way here is to make sure your panel brand equals your breaker brand.  Wrong brands/types will seem to snap in, but don't properly mate and will arc and burn up the buses. 

This isn't scamming you, we're not name-brand-obsessed, if you want to use a GM/Delco brand brake pad made for an F150 on your F150, have at.  That's just not very common with breakers.