Is she particularly powerful, though? She's extraordinarily talented, very knowledgeable for her age, and has more raw power than anyone in her year including Draco; but Rita is more experienced, and most importantly older - it has been repeatedly pointed out that HP lacks the raw power for something-or-other, and the twins are far stronger than he despite not being particularly talented. It seems that Rita should have an edge in the "raw power" department, and I'd expect this effect to key off raw power.

Note that it's also sufficient to assume that Quirrel and/or Mary's room can suppress this effect.