If you were to grab a handful of cards from the box of 500, they would be packed in sets of 3 and 4, as follows...

Group A of 3 cards has cards 1, 2 and 3. This is followed by another set of Group A cards.
Then, Group B has cards 4, 5 and 6, followed by another Group B set of 3.
Then Group C, with cards 7, 8, 9 and 10, followed by another Group C set of 4.

Then the pattern repeats.