"Sommige" means some, but not all. It points to a subset of a larger group (in this case, all apples).
"Wat" can mean some, a few, a couple of, a bit of. It is simply an undetermined, small amount.
- Sommige honden drinken wat water - Some (not all) dogs drink some (a bit of) water.
Does it more closely mean: Some of the apples are sour (even though they are all in the same batch of apples)? Or: Some (varieties of) apples are sour (as opposed to other varieties)?