A new kind of math?

Ok, so my son's birthday is coming up. I was thinking about getting a novelty cake from the local grocery story bakery. The conversation went something like this:

me: What are your sizes and prices?
employee: We have 1/4, 1/2, and full sheet. The prices are *gives prices*
me: Hmm, ok, how much does a quarter sheet feed
employee: We don't have a quarter sheet. We have 1/4 sheet, 1/2 sheet, and full sheet
me: *blinks at her* You don't have a quarter sheet, but you have a 1/4 sheet?
employee: *nods*
me: *decides not to pursue that line of thinking* Ok, so how many does the quar..I mean, the 1/4 sheet feed?
employee: 6-8 people
me: Hoe many does the 1/2 sheet feed?
employee: 30-35 people
me: *eyebrows draw together at the mathematics of that* Ok, so the quar..I mean the 1/4 sheet feeds 6-8 people, but the 1/2 sheet *thinks to self how that's just 1/4 sheet x 2* feeds 30-35 people?
employee: *nods*
me: *scratches head* ok, thanks for the info, I think I'll pass on it this time. *heads to the cake mix aisle and decides to do it myself since the baker can't do basic math and might f$#* up the ingredients*