But what is this unintuitive way that the deck works?

My best guess is that it's one of two approaches.

1. The probability of any individual Frequent card is twice the probability of any individual Standard card, etc. So, say, a deck with 6 Standard cards and 2 Frequent cards would assign 10% to each of the former (total 60% among the Standards) and 20% to each of the latter (total 40% among the Frequents).

2. The probability of the Frequent bracket as a whole is twice the probability of the Standard bracket, assuming at least one card exists in each. So in a deck with 1 Standard and 6 Frequents, the Standard would have a 33.3...% chance, and the Frequents would collectively have 66.6...% (each one has an 11.1...% chance).

I feel a little dumb for not finding the info here; it must be somewhere on the wiki. Right?