Are you talking about FEV1 values, liters vs percentage? I like to look at liters. Different clinics use different predicted values (amount of a "normal" FEV1 for someone your sex and age) But your own liters will never change, no matter where you are. A liter is always a liter, but if you change the predicted value, the percentage will change. Does that make sense?
For example, my predicted FEV1 is about 3 Liters. My FEV1 is currently 1.5 Liters. So my % is 1.5L/3L = 50% FEV1. But if I'm at a different clinic, or my clinic uses a different predicted value, my % might change. There are different "predicted" charts. Hankensen (sp?) is one, I know there is another popular model out there. Based on those different "charts" my FEV1 percentage can change, but it will always be a certain number of liters. So if I still blew a 1.5L on any given day, but they use a different predicted model, say 2.9L Liters my percentage changes to 1.5L/2.9L = 52% (see how my percentage makes it look like I got healthier, by changing the predicted model?) But I still have a quantitative FEV1 value of 1.5 Liters.
I hope that helps. I'm sorry if I've confused you or answered a question you didn't ask.