Ib is Mary's best friend, so she's mean and cold to Garry, who is always with her. That's pretty childish, but we like that about her. This tag is for when Mary is like that.
It's probably your imagination that the friendlier Ib and Garry get, the more Mary's interference escalates.
Hey, you in the peanut gallery! Don't tell us there needs to be a point!!
As an aside, illustrations that receive this tag mostly involve Mary cockblocking Garry when he and Ib are trying to have a moment, as is written above. It's a little different from other "____ As Usual" tags, where the character in question is often being mistreated.