Progression from 2007 to 2013

I'm always surprised and a little envious when I look at other artists progressions. I see this huge leap in their skills from one year to the next. I was kind of disappointed when I put my progression together, because while I do see an obvious changes from the first two years to what comes after, it doesn't seem to improve much after that. These past two years I've branched out into sculpting polymer clay and paper cutting, which was an exciting change for me, but I've looked back at some of my art and noticed that I have not been consistently improving or even reaching a plateau, because if some cases it appears that I have regressed and unlearned things I could do before. Why does that happen?