I think your missing the entire point. The UFC isnt the one allowing the extra .9 the Quebec Athletic Commission does not use decimals but merely rounds down. A UFC employee came over to Diaz to make sure he was aware that the AC did so because its not normal in other places like Nevada or California. Diaz can complain for not knowing it earlier but I would think it would be he and his teams responsiblity to understand the rules in the jusrisdiction they are fighting in. In good faith the UFC likely should have let him know sooner but regardless its not there call.
My problem is that there is no concrete evidence that the Canadian commission has implemented this .9 lbs allowance? Without evidence, it's probable the Canadian commission made it up for GSP's benefit.
Maybe someone could shed better light on it instead of sweeping it under the rug.