I bet you had a lifter that was starting to seize up. At first, it was keeping the valve open. But as the cams lobes beat it up, eventually it began to compress properly and your compression is back.
Lifters often will go hard when they sit for a long time like that. When you say it's been sitting for 5 years I'd say you're lucky you don't have some completely seized lifters. ;-)
I doubt your valve was stuck. I suppose it could have been. Guess it doesn't really matter. You've got compression so this would be the absolute best result possible...fortunate!
