The labels on the apples my wife eats bear the legend: "Marlene Sudtirol Mela Alto Adige IGP". When I commented that I though she always ate Granny Smiths, she replied "well, they came out of the Granny Smiths bin".
Research came up with this:
About Marlene | Marlene English
So the variety theory is not just blown out of the water, it's shot down in flames!
It depends on the country they are sold in. In some countries, the labels do have some small numbers on them that allow people to type those numbers in at checkout when the fruit is weighed to get the correct price.
Sometimes the stickers are just the brand name of the company that markets the fruit. Like any other company, they want to have their brand be out there and recognisable. With fruit, it's tricky, as there is no guarantee that the retailers that sell the fruit to the consumer is going to label the container the fruit is in with the brand name of the company that produced/packed the fruit. So putting a sticker on every piece of fruit ensures that the person who eats the fruit and likes it is more likely to buy fruit from the same brand in future.
The variety/cultivar of the fruit is only one part of the quality of the fruit. There's taste that's affected by the soil, the way the fruit is cultivated, whether or not it's treated with specific fertilisers, pesticides, whether it's washed properly, etc.
You might know that you like Golden Delicious apples and be able to recognise them by sight, but that in particular, that cultivar from a specific producer is your favourite.
Labeling the fruit is is done mechanically, it's not some person sitting there sticking a label onto each apple. Most fruit packers of any kind of size and scale use automated conveyors where every fruit falls into a little cup on the conveyor which weighs the fruit individually. It then passes through a scanner that visually scans the fruit to check its colour and for any obvious blemishes. A machine applies a sticker to each piece of fruit. The conveyor then passes over a set of other conveyors running perpendicular to the initial set and the cups open, letting the fruit drop onto specific conveyors so that fruit of the same size and colour end up in the same area where people pack them into boxes or they fall into bags or bins. Undersized, oversized or "ugly" fruit fall off the end of the conveyor to be made into juice and/or fertiliser. This way, the fruit you buy in a box are all uniform in size and colour. Pretty ingenious system.