It could be more compressed, or it could just be a slightly different grade of plasic that is more dense. Compressing a solid like plastic is incredibly hard and would take different machinery than normal moulding (not to say it isnt the machinery they are already using and just not using the same functions of the machines with everything they do), there could just be air mixed in with the lighter ones though. There are many different types of PVC for example, seveal thousand different grades if not more, each with slightly different properties (density, elasticity, flexibility, heat resistance, etc.)
Mixing in air can make things strange as it isnt controlable as precisely so some BB's would weigh less or more than others, so chances are its slightly different types of plastic.
(Just had a test this week on materials and manufactuing processes)