Let's say you get a legendary reward card.

There are 2 legendary cards currently out, the odds you'll get each is 50%. That means there'll be quite a lot of those two (and why the 1 that was out till now is so low-priced, it's much more common relative to the card pool than non-rewards legendary cards).

Now let's say there are 20 legendary reward cards. You only have 5% to get each one. Each legendary reward card is now 10 times as rare.

It works the same for common cards and all other rarities as well.