'Non-standardised' basically means that no Cream has been skimmed off the top of the Milk. Or, to put it another way, all the natural goodness that the Cows produce has been left in.
Why would cream be removed from Milk? Well, Cream is valuable; it is always more expensive than Milk. This is because Cream makes up only a small percentage of Milk.
The 'Standard' percentage of Cream that must be present for Milk to legally be called 'Whole Milk' in the UK is 3.6%.
Ayrshire Cows produce Milk that contains between 4.5% and 4.9% Cream. This varies with the season: less in Summer when the cows are in the Pasture eating protein-rich grass and more in Winter when the Cows are in the Byre eating Silage.
Removing this seasonal variation is actually given as an argument for standardisation! In actuality though, it was purely for the convenience of the industry rather than the consumer. For one thing, labels detailing fat percentage need not be changed if the fat percentage remains constant. We think that removing this seasonality is a bad thing in principle, further disconnecting the consumer and producer.
By standardising Milk, Big Dairy get to sell you the same thing twice, even skimming off the top of blue-top 'whole' milk. Standardisation is also why red-top 'skimmed' milk exists in the first place - they get to remove all but 0.5% of the Cream! No-one asked for Milk to be less healthy, less tasty and more like chalky water, but under the myth of 'fat is bad' the concept was pushed on an unsuspecting public.
All fats are not the same. It turns out that the healthiest type of Milk is Whole Milk. This is in part because their are some Vitamins that are non-water-soluble and can only be absorbed by the body when dissolved in fat. Vitamins A, C, E and K all fall under this heading.