You'll find that most folks who have "inconsistant" carbonation when we pursue it further it turns out they're opening their bottles too soon. It's quite easy to blame or assume it's because of something, rather than the truth.
I personally believe the whole "priming sugar didn't get mixed" argument is BS, if you put the sugar solution in the bottom of the bucket and racked your beer, then it couldn't help but be mixed. You're putting 2 tiny cups of liguid into a vessel and dumping 5 gallons into it and the beer is rising as it fills the bucket...believe me, it is mixing.
Most of the time when a beer is acting weired, it's just that it's not fully carbed yet. And if you're below 70, or were below 70 for any period of time during the 3weeks, then the beer hasn't fully carbed yet.
Inconsistant carbonation, simply means that they are not ready yet. If you had opened them a week later, or even two, you never would have noticed. Each one is it's own little microcosm, and although generally the should come up at the same time, it's not an automatic switch, and they all pop on.
A tiny difference in temps between bottles in storage can affect the yeasties, speed them up or slow them down. Like if you store them in a closet against a warm wall, the beers closest to the heat source may be a tad warmer than those further way, so thy may carb/condition at slightly different rates. I usually store a batch in 2 seperate locations in my loft 1 case in my bedroom which is a little warmer, and the other in the closet in the lving room, which being in a larger space is a tad cooler, at least according to the thermostat next to that closet. It can be 5-10 degrees warmer in my bedroom. So I usually start with that case at three weeks. Giving the other half a little more time.
Bottom line, it's not that the sugar's not mixed, it's just that they all haven't come up to full carb yet....Three weeks is not the magic number for finality, it's the minimum time it takes....