I thought the same way as you a few months ago so I created a little routine that captures the card numbers of the daily specials. I have data for 148 days so far and 8 cards a day gives a total of 1,184 specials in that time. In that period 491 different cards have been offered as specials, 251 of those more than once.

Some cards do come up much more frequently (cards 18,68 and 351 have each come up 11 times in that period), but it's not always the same cards: in 4.5 months over 2/3 of the available cards have appeared as a special. You do see 'clumps' of the same model where several of her cards are available over several days, but not always the same model or the same cards.

As to why: I'd have thought it makes sense to offer the product that's moving slowly at a discount to try and increase sales. So, if you're a fan of the following you'll probably not have to wait long for her cards to come up on special:

Elya
Zoe
Mitsouko
Dorothy B
Maggie
Liliane T
Felina
Carol G
Lucie T
Ting

Even so, not all of a model's cards are frequently available. Take Elya as an example. Her six different cards have been on offer a total of 46 times:

card 17 offered 9 times
18: 11
19: 2
20: 7
21: 8
22: 9

Just don't hold your breath for Camilla: in 148 days none of her cards have come up as a special.