You severely miscalculated the set you said was a couple dozen low. The stats of it are 511/756/620.

DET (511-218)*0.139 = 40.727
CRT (756-354)*0.162 = 65.124 (not sure how you got 48 here?)
SS (620-354)*0.104 = 27.664 (again, not sure how it was 20)

Which adds to 133.515. Still a far cry from Dervy's set, but a lot closer than your sheet shows.

Dervy's set definitely looks to be pretty ideal, imo, since it picks up the thing about the highest weight set ( http://ffxiv.ariyala.com/RL2T ) that really irked me and made me wanna look for an alternative - 357 det is depressingly low. Working it out the way you did makes it spit out 138.557, which does beat out Dervy's, but the 357 v 484 det is significantly less consistent, imo.

Also Ariyala doesn't calculate the best in slot thing including food. The buff to crit from Pipira Pira isn't factored into the final weight calculations on the site, so it doesn't actually calculate true best in slots, factoring food in.