Unequal pay between grad students, enforcing a more egalitarian policy
The more competitive and prestigious the school, the easier it is to be egalitarian, because you can always get students to enroll there. When you have trouble attracting students, incentives that may lead to an unequal playing field may be needed to get better students.

For instance, when I was applying, there was about a 50% spread between my lowest and highest offers. However, some of this was handled not by awarding an RA on top of an external fellowship but a “merit prize” of several thousand dollars as a reward for bringing in the fellowship. That might be a way to offer something substantial for people who have their own money without making things too far out of balance.