When I still used to bottle, many moons ago, I would get that too.
I think it has a lot to do with the distribution of the priming sugar. When the sugar is added to the bottling bucket, unless it is completely and thoroughly incorporated, some bottles will be under primed and some will be over primed. Constant stirring is the key I think.
My buddy who still bottles goes one further and adds the sugar to some sterile water and heats it up until it is completely clear then adds that to the bottling bucket. Liquid vs solids and all that. He still stirs it frequently to avoid the heavier sugars to sinking to the bottom.
Cheers,
knewshound
I have always done what your buddy does using corn sugar. One cup water with 5 oz corn sugar. Slow boil sugar in the water, let it cool and add that to the bottling bucket then rack the beer into the bucket to get a good mix going.
For my 1 gallon Hard Ciders I have been using Coopers Carbonation Tabs in the bottles. They work well.