Colleges get "tricky" with this. One school told my son they were making him the same offer as a kid who committed to that school last year...but was drafted in the first round and didn't make it to campus. But that kid was an in-state student and my son was an OOS student. True enough...but it was a difference in about $15K per year. Technically, I guess it's true...but in the the context of the conversation, I viewed it a little shifty. He ended up saying no (not for that reason) and I'm glad.
I think when schools were talking about percentages, it was based on Cost of Attendance. At least that was my son's experience. Some gave dollar amounts and others gave percentages. At the end of the day, it's what is left to pay and the value of the institution that matter most, I suppose.