9×9 pixels =total 81 pixels. you are right.

but how do you want to access individual pixel?

straight by numbering each pixel individually ? then from 0 to 80(tot 81 pixels) , you require 7 bits to address individual pixel.

but when you use row and column address for referring individual pixel ,
row requires 4 bits and col require 4 bits.(tot 8 bits)

since it is easy to refer individual pixel by (row and col ) addressing , we usually use that method.