well, let's see the 10-pack is $15.37 BUT each of those 10 bars are 2.1 oz, so for the whole package you're getting really 10 * 2.1 = 21.0 oz.
the 12-pack costs $15.35 BUT each of those 12 bars are 1.4 oz, so for the whole package you're getting 12 * 1.4 = 16.8 oz.
which is the better deal? which one gives you more ounces for the money? after all, you're going to eat the bars, so you want more ounces, not more bars.
so clearly you'd want to get the cheapest one with more ounces, so at $0.73 or 73 cents for each ounce, the 10-pack is the cheapest.