More than likely then, they are not actually fully carbed yet. And your 2 hours in the fridge is STILL pretty warm, and the co2 in the head has not been fully absorbed into solution in the cold, what little there is. On the other hand, the longer chill IS absorbing the limited amount of co2, and since it's a small amount it is no appearing carbed.
Remember that the 3 weeks at 70 we recommend is just a rule of thumb and not etched in stone, and that is only for beers that are around 1.060 og or less. Bigger beers do take longer. I have had stouts and porters that have taken 6-8 weeks, and a 1.090 belgian strong that took three months to achieve carbonation.
Like I said, I don't think that even at 3-4 weeks your beers are fully carbed to begin with...give them a couple more weeks above 70, and agitate them once to lift the yeast back into solution...and check back on them in another week or two, they should achieve full cab levels by then, and chilling them for the 48+ hours we recommend should net you perfect beers.
