To recognize the difference, you need to keep the revenue streams separate. In the movie analogy, consider the difference between flat tickets at $5 and tickets that give TheaterBucks rewards at $6.

A $30 allowance allows you to buy 6 normal tickets establishing a base rate of $5/ticket.

Alternatively, your $30 allowance would allows you to buy only 5 premium tickets at a worse rate of $6/ticket.

But, in the RewardProgram, purchasing a $6 ticket includes a $2 TheaterBucks voucher, good towards future tickets at the normal price. With your 5 tickets you also got total of $10 in TheaterBucks, which allows you to get 2 more tickets.

If TheaterBucks were regular dollars, then you spent $30+$10=$40 for 7 tickets, at a less-than-ideal rate of $5.71/ticket, which is worse than $5/ticket. But that is the wrong way to look at it. Actually, for your initial $30 allowance you were ultimately able to get 7 tickets, at an improved rate of $4.28/ticket.

This is what we face with the various chests in this, and previous quests. Substitute "candies" for "dollars", "flags" for "tickets" and "bonus candies" for "TheaterBucks".

Suppose you have an allowance of 500 candies and are faced with a regular chest at 20 candies/chest and a chest that gives bonus candies at 24 candies/chest, both offering a single flag.

If you go with the 20 candy chest, you can open 25 of them, getting 25 flags, at a base return of 20 candies/flag.

If you go with the 24 candy chest, you can only open 20 of them, getting only 20 flags, at a worse rate of 24 candies/flag (and 20 leftover candies, but who's counting. For the rest of the scenario, round down).

But, the 24 candy chest offers TheaterBucks with a 10% chance of 75 bonus candies. This could be thought of as getting 7.5 bonus candies with each 24 candy chest. 20 chests = 150 more candies, allowing you to open 6 more premium chests, getting 6 more flags. Oh, and those 6 chests gave 45 more candies, allowing you to open another chest, for another flag. (Elvenar is better than my TheaterBucks program, in that you can get rewards for using your rewards.)

You could say that you spent a total of 480+144+24=648 candies and got 27 flags, at a less-than-ideal rate of 24 candies/flag, but that's forgetting that you started with only the initial allowance of 500 candies. Actually, for your initial payment of 480 candies you were ultimately able to get 27 flags, at an improved rate of 17.8 candies/flag.