I can guess it's a (functioning, presumably) trap for the inattentive but the pricing scheme on your menu shouldn't be making me look at it funny, dagnabbit!
There are some examples where adding in additional
crappy choices that no logical person would ever make can change people's decision making about the remaining choices. There was a TED talk about this.
One example was for a magazine subscription. The choices were something like
(1) $30 digital subscription
(2) $125 print subscription
(3) $125
print plus digital subscription.
Clearly, choice 3 is a dominant choice over choice 2. People think "well I might as well get the digital as well since it costs no more". In this scenario, about 2/3rds of people express the preference for choice 3. However, tests were done with removing the "useless" choice 2, and suddenly, 80% choose choice 1 instead. i.e. nobody capable of reading ever picks choice 2, yet the presence of choice 2 changes how we evaluate the quality of the remaining choices.
There were a number of other examples given. One was
(1) All-expenses-paid Rome holiday for 1 week
(2) All-expenses-paid Paris holiday for 1 week
Asked which one to choose, respondents were pretty much 50/50. However, they did the experiment with a third choice, one of either:
(3a) All-expenses-paid Rome holiday for 1 week. Except coffee with breakfast costs 2 euros.
(3b) All-expenses-paid Paris holiday for 1 week. Except coffee with breakfast costs 2 euros.
In other words, they added the smallest and least-consequential negative thing that you could add to an all-expenses paid trip to Rome or Paris. However, test subjects shown (3a) suddenly love Paris with coffee, more than paris-without-coffee, but paradoxically, also more than Rome-with-coffee. If you showed them (3b) instead, which is rome-without-coffee, then they love regular Rome and don't like Paris any more. This is how fucked we are at making decisions.
Another example was they showed two photos of guys to women, got them to pick the most handsome. Again 50/50. But then they made "ugly" versions of both guys. Call the guys Adam and Bill. If you saw "ugly Adam" then regular Adam looks better, even better than Bill, and if you saw "ugly Bill" then Bill looks better, even better than Adam. So the take away is that your wing-man should be a slightly uglier version of whatever you look like.
Now let's consider the BK menu.
First, notice that there's not much difference between the 4 piece pack and 10 piece pack in price. So people would go "might as well get a 10 piece pack then". But also notice that
two 10 piece packs have the same amount as a 20 piece pack, despite being much cheaper. Now, not only have you convinced yourself that a 10 piece pack is better value than a 4 piece pack, if you
only buy one pack you start to feel you're
missing out because you didn't get 20 pieces. So, maybe you only wanted 4 pieces to start with, but you've talked yourself into buying 20 now. i.e. nobody in their right mind ever picks Option 3, but it's still there for a reason, because it makes the other options look better, and causes people to buy extra so as to not "miss out" on the "saving".