a little question?

hello. im new here although have suffered with asthma for many years. this forum seems an amazing place and i just wondered if you would be able to help me with something that has confused me slightly today? i saw my nurse at the hospital today and had some spirometry. my peak flow was amazing, probably my best ever, at 510 and then 490!!! what really confused me was the fev1/fvc was then 0.7. how can i have such an amazing peak flow and awful fev1/fvc???? thank you in advance for any replies! x