Amazon VPC n^2 -4 IP Addresses? CIDR Block

I was in the process of creating a new AWS VPC for my instances. However, I noticed that when I used CIDR Notation to create the VPC & Public Subnet, AWS indicates that I have n^2 - 4 (where n is the # of bits) available IP addresses? Why is this?

I understand that when n^2 -2 occurs it is usually to remove the case where bits are all 0s or all 1s. But am not sure why it is - 4 in this case.

Here the /28 indicates 11 IP addresses available when I expected 15 or 13, and 251 when I expected 255 or 253