According to the Connecting Threads website, MasterPiece is a 2-ply thread.
Masterpiece is a 3 ply thread, the pre wound bobbins of Masterpiece are 2 ply.
Mercerized cotton is great for hand-work, not so great for piecing by machine (and would be almost impossible to machine quilt with it). It is a chemical treatment that makes the thread thick & crisp. I used it once before I knew better & it gummed up my machine & gave me terrible results.
You used Handquilting thread with a coating, not mercerized cotton thread. Essential threads are not crisp and will not gum up your machine.