Hello and welcome to the forum!
The reason that some items don't stack is because they have different decay timers. This is a game function.
Meat, fruit, vegetables and grains don't stack naturally. It is all based on how "old" the item is.
Some foods decay faster than others. If you've killed an animal, and then 1 hour later kill another, the meat would not necessarily stack.
The game will group the meat by "time killed". This is to encourage processing and using the meat you get quickly, before it goes bad, as in real life.