What determines which Mature Content warning Steam gives you?

So when browsing Steam games, I've noticed two distinct "Mature Content" warnings:

First, one that asks you to enter your birthdate. It looks like this:

However, recently I've started seeing a different warning, which says "Content in this product may not be appropriate for all ages, or may not be appropriate for viewing at work", and then simply asks you to continue or cancel. It looks like this: (the "continue" and "cancel" buttons were cropped out accidentally)

What does Steam use to determine which to show?

(Related side questions: Are there other possible warnings? Why do some M-rated games not get either of the warnings, like The Witcher 3?)

Those two are the only warnings. The difference between the two is that one has an age rating and the other one has not.

You can see the second message for "Early Access"-games, pre-orders, betas and demos which have no age rating yet.

Sometimes a DLC which adds blood and/or gore has the second message too, because the game itself is not rated M, but the DLC would be. A recent example is the Blood for the Blood God DLC for Total War: Warhammer.