They just haven't jammed on you yet.
Don't be too quick to blame the gun and/or ammo before realizing there are 3 fingers pointing back at yourself.
One of the regular patrons of the local pistol range here refers to this ammo as "Winchester Super-Jam" It does just as well (just as bad?) in the Mark III's.My Ruger Mk-II doesn't seem to like Winchester Super-X ammo