We can make an estimate, even without knowing the programming, I think. Let's assume that each rare collectible has an equal chance of showing up when the chief points one out.

There are 16 different rare collectibles (four of each type), so each should have about a 6.25% chance of showing up - 1/16. This number gets higher if you already have some of the rare tablet pieces (because these can only be collected once) - it becomes 8.33% if you have all four rare tablet bits - 1/12.

But that doesn't mean you can't get the same one two or three times in a row .

(Actually, I don't think this changes if the game randomly selects which type of collectible should appear first and then chooses from the four pieces. But I could be wrong.)