There have always been continuity errors regarding a drone's transition into a warrior. Despite the fact that certain victims were actually being chestbursted & drones were emerging during the time that the marines were investigating Hadley's Hope, not a single drone was spotted.
In Grid's case, the chemical that was pumped into the queen's body which caused her offspring's lifecycles to advance at an accelerated rate could have possibly aided him in "moulting" into a warrior at a much faster rate. Although I suppose the chemical would have affected the other aliens in the same way. There's another continuity error in itself, lol.