I firmly believe that the not mixing issuse is a myth. The mere racking is enough to integrate the two fluids just fine.
You're talking 2 little teeny cups of liquid being overwhelmed by 5 gallons of liquid of pretty much the same density- It's impossible for it not to be mixed up
We're not talking oil and water, we're talking sugar water and alcoholic sugar water....it's not much difference.
Some beers can take 6-8 weeks or more to fully pop and the fact that a few bottles may be carbed only means that those were a little warmer than the rest of them, NOT that some got more sugar than others.
What folks seem to forget is that when you stick the beer in the bottles, you are sticking them into separate little fermenters.
Each one is it's own little microcosm, 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.
You can split a batch in half put them in 2 identical carboys, and pitch equal amounts of yeast from the same starter...and have them act completely differently...for some reason on a subatomic level...think about it...yeasties are small...1 degree difference in temp to us, could be a 50 degree difference to them...one fermenter can be a couple degrees warmer because it's closer to a vent all the way across the room and the yeasties take off...
Someone, Grinder I think posted a pic once of 2 carboys touching each other, and one one of the carboys the krausen had formed only on the side that touched the other carboy...probably reacting to the heat of the first fermentation....but it was like symbiotic or something...
With living micro-organisms there is always a wildcard factor in play. Two complete fermentations (and bottle conditioning is just another fermentation) can behave differently due to even the slightest change in enviorment, especially temps.
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....
I've bottled THOUSANDS of gallons of beer, I've never mixed, and I've NEVER not have every single bottle in a batch not be carbed eventually.
One of these days I'm going to make a perfectly clear bottling bucket, color my 2 cups priming solution with food coloring, point 3 different cameras at it from three different angles, rack my beer onto it, so folks can see the color dissipate into the beer, and finally put this nonsense to bed.....
How long has the beer been in bottles? More than likely they're at the cusp of carbing and in another week they'd all even out and be fine.