The results of the ramp test are calculated by multiplying your best 1 min avg power by 0.75 (I think, it could be 0.74). The backpedal at the end would not have affected your results. But if you look at the last step of your test, you can see that you weren’t quite hitting your power target. So even though you lasted longer your best 1 min avg power may have been a hair lower. This would also raise those other duration PRs because you held a similar peak power but for longer.
But 251 vs 256 is well within the margin of error of any test when you take into account power meter accuracy, nutrition, sleep, motivation, etc. So maybe you had a tad less stress last year or ate more carbs that day or it was a more sunny day and you just felt better. So I wouldn’t worry about it. And I certainly wouldn’t call it “crushing”. I would move forward with 251 or manually bump it up to ~255 if you feel that is more accurate.