I've been counting some numbers on the slot machine.
I was out of credits earlier today and discovered you can still spin the slot wheels - you don't get anything regardless of your result (no credits for stars, cards don't reveal, no free cards) but it meant I could spin a bunch without paying and get some probabilities to know if it was worth playing.
I span 200 times and found that each wheel appears to have a 10% chance of the "best" result (a star for the first four, the "Free" result for the last assuming you're betting 5 credits). Further, the probabilities seemed independent - certainly I got as many 1* and 2* results as I'd have expected with independent 10% chances on each. I didn't get any 3* results but over 200 spins with those probabilities, the chance of getting a 3* result is about 50/50, so no big surprise.
It all looked very fair, and examining the results a little more makes sense for it as a gambling "The House Must Win" result (the credit rewards mean you get on average back about 0.8 credits for every 1 credit you put in - although those numbers are dependant on being lucky/persistent enough to get the 3* and 4* rewards eventually; if you never roll those you only get a reward back of 0.5 for every 1 in).
And as I said before, the FREE result on the final wheel came up about 1 in 10 times too. Counting the cards I 'won' over my 200 test spins, I would have got 76 new cards if I'd been playing for real. Given the number of credits also won as an offset to the cost, that came to spending a net 475 credits on 76 cards - an average of 6.75 credits per card. Yes, the cards are randomly picked, but that's a pretty good price nonetheless!
So I bought some more credits and started playing.
The good news: the stars seem to stay fair: about 1 in 10, seemingly independent.
The bad news: the discount wheel strongly appears to have different probabilities when you have actual money to spend.
I have just spun (and recorded my results) for 70 consecutive spins with actual credits - and I haven't rolled a single instance of free cards. If the 10% probability I found while spinning tests were still in effect, the chances of my luck being that bad is 0.18%. Not 18% - 0.18%.
Which is a bit of a bummer - not to mention *****. I'm surprised the wheels would be programmed to change probabilities in the edge case of not having credits.
It's possible that there's something else going on - when you bet with 0 credits available, the page automatically goes to the Add Credits page but the wheels keep spinning until you go back to the Slot Machine page. If you tab away when spinning normally, the last dial also spins until you tab back. Maybe the probability of that last dial landing on Free also gets affected somehow by the different times it spins for?
Or maybe I'm just that unlucky. Ah well.