Its pseudo random its not exactly rigged.

To demonstrate this picture you have a hat and inside the hat there is a bunch of vouchers but only 6 different ones.

You stick your hand in and you say to yourself wow for 6 items there are a lot of vouchers in here and thats because there are actually:

8 solo scrolls (Equivalent of rolling a 6)
8 pt scrolls (Equivalent of rolling a 5)
1 1000 pet feed (Equivalent of rolling a 4)
3 50% hp scrolls (Equivalent of rolling a 2)
3 SES (Equivalent of rolling a 1)
4 mystery cards (Equivalent of rolling a 3)

Then you have another "hat" for the mystery card where there are

5 go forward 1 space
1 go backward spaces
3 go forward 2 spaces
2 go back to start

And if you get a go backward spaces voucher there is more pseudo randomness relative to how close you are to the start (because we dont want you to get an easy golden key AKA 4)

You have a chance for all of these things but its programmed so its less likely to occur just like the odds of getting blackjack or the odds of getting 5 under 20.