Armored Ascension‘s ability that grants +1/+1 to the enchanted creature looks at the number of Plains controlled by the controller of the Armored Ascension, not necessarily the controller of the creature that that Armored Ascension is on. This can matter if a creature is enchanted by Armored Ascension and then this creature is ‘stolen’ by something like Mind Control. Another situation where this can happen is when an Armored Ascension itself is moved to another creature that is controlled by a different player. This can happen while the Armored Ascension is already in play, or when it is a spell on the stack by having its target changed by something like Swerve.