And that's my point - if you want to measure your improvement at parkrun, focus on your finish time. Try and increase your age graded percentage. Don't feel discouraged if you never finish first in your age category - you need to recognise that while someone may be in the same age category as you, they've also have been running since they watched the Olympics on television when they were in year three at primary school and begged their parents to let them do Little Athletics.
Last of all, always remember that despite people's competitiveness, parkrun isn't a race. You can't win it, so if you come first finisher, that's nice, but the only thing it really means is that the barcode scanning queue is a bit shorter for you than it is for the people who finish after you.