I think that if you tell them that you will be away from Thailand at the renewal date then they let you do it earlier. I read a post once on Sawatdee I think about somebody renewing it 4 months earlier for that reason but it was before the 3 month deposit rule.
I know for sure that immigration offices have different rules, like embassies or consulates around the world. Sometimes you will even get different answers from different people within the same office !
Anyway I'll let you know next month about this 30 days advance renewal, it might be useful for some who go to Suan Plu. I believe you are right, it must have been Suan Plu own rule before, may-be due to too many applicants coming at the last minute.