Coopers Dark Ale: beer from the same batch is carbonating at different rates, why?

I did a coopers dark ale a while ago and it had the most head retention I have ever had from a home brew. It was like an ice cream when looking at it. I wish I had a picture, but I don't, unfortunately.

Anyway, I have done another one, this time with some chocolate malt and crystal malt, steeped. It has been in the bottles two weeks, and some feel tough to the touch, like they're carbonated; others don't. I opened two, for experimentation purposes, to see if they were very different, and they were. One was under carbonated and the other over carbonated. I have ruled out infection because they taste fine and from the same batch. However, why would this be? One starts to form head in the bottle, so you have to pour quickly. And the other is flat, nearly. I definitely put the same amount of carbonation drops in because I am anal about that and am sure I got it right.

The only explanation is that the one that is carbonated already is the one that I squeezed out the barrel and got a lot more sediment in, i.e. the one from the bottom of the fermenter that most people would probably throw away.

I have never had such extremes in beer, despite the fact I have bottled to the sediment every time.

That said, my questions are these...

Is it the dark ale producing a lot of head?

Is it the extra yeast that probably got into the bottle when bottling?

Is it anything else?

P.S. I made sure it finished fermenting because I checked it all the time and have never had a problem before.

Cheers

Regards

Rob

What temperature were you carbonating at? How long?

It's possible if they were carbed too low that most of your yeast are dormant. The bottle with more sediment had more yeast in it, so a great number still working through the cold.
The few yeast still in suspension of even a crystal clear beer will still be enough to carb, but it may just take longer.

I'd bring all the undercarbed bottles up a few degrees and let them sit for a few more weeks.

They were in my room, all at the same temp - about 20 Celsius. I know it's a bit warm, but I don't have a choice in my house - no room.

I guess it was down to the more yeast speeding up the process. I am not sure I will steep grains in my dark ales again.

Yep, it's due to uneven levels of yeast. Relax and give them time - they will even out.