It's in one of the novels. I'll figure it out eventually. Point being, the title isn't bestowed lightly. Either it's related directly to ability or position within the order or both.
And for all we know, the ancient jedi order may have had up to three or more grandmasters, seeing as they did have three branches of the order back then (Sentinels, consulars, guardians, etc,) I'd also like to point out that the title Watchman was bestowed to jedi masters who guard and observe certain areas, such as a planet or even a solar system.