The reason is potting mix is sold by the volume is the moisture varies between batches and over time with storage, significantly affecting the weight of the bag. A 10% reduction in moisture content, which is easily possible, may change the weight of a small 25L bag by about 1-2kg depending on the mix composition,
The reason the moisture content varies between batches is because composting and making the potting mix is done outdoors. The final mix moisture is dependent on the weather conditions prevailing during composting and making. During dry times, the mix can be significantly lighter than in wetter times.
The other factor is pots used for potting mixes are also measures in litres. If one buys 25L of potting mix, it should more or less fill pots with a overall total volume of 25L. If one buys a bag of potting mix which is say 10kg, one doesn’t really know how many pots it will fill or if it will be sufficient for the planned gardening work.
The organic matter in a potting mix should be stable and shouldn’t breakdown much in the month(s) between bagging and use by the consumer. The volume however is loose volume when the potting mix is added to a bag. If the bags are placed on pallets and stacked, there is potential that the mix will be compacted. This loss of volume through compaction can be overcome by loosing up the potting mix when the bag is opened and before using.
The bags of seed raising mix and potting mix I buy have small perforations to allow “steam” to vent and a small amount of air in. They are stacked on pallets with these outwards. I raise seedlings. When I open the bags I go through a process of fluffing it up, balancing the moisture levels for optimum germination.
The bags are in litres, but they have a ‘heavy’ symbol suggesting 2 people to lift. I guess if they said it was 25kg and it sat in the sun venting moisture (and weight) you could complain that you were undersold on content.
Change in moisture. When storing for some time the water evaporates, making it lighter.
Loss of water reduces the capillary action between particles making the mix looser or fall apart. A bit wet and dry sand. (Note: there are also other forces which can affect adhesion of particles as well, but generally in fine organic materials, it is water which causes the adhesion).
Just be cautious that as it dries out it becomes dustier. The airborn dust can carry pathogens from the potting mix. It’s best to use it fresh. If unavoidable a light spray of water and shake when stored will help to maintain moisture.