I experienced the same thing. I took spin classes 3x a week and had really improved there and expected to be much better than I was when I quit for the winter. I was dying at first but felt like I was doing really well after a couple of months so surely I'm improving, right?
I got back on the bike in Feb and there was virtually no improvement. But, the people that were around my level in November and had done little or nothing for 3-4 months were off the back in a hurry.
I guess there's no substitute for the real thing, but at least I maintained what I had and can keep improving instead of working to get back where I was. I have noticed that the hills don't kill me as badly as they used to and I can stand up a lot longer than before, so it wasn't a total waste.