Why is the number 78 x 48 x 71?

I thought it should be:

1 layer of base (78 x 50 = 3900)
2 sides (50 x 72 x 2 = 7200)
2 faces - front and back (78 x 72 x 2 = 11232)

Total number that touch inside of the box = 3900 + 7800 + 11232 = 22932

Ooops,
Touch the box should be 288000-265824= 22176

@Wil
Your last step should use
78×71×2= 11076
Because, base layer already count in your 1st step.

So, total
11706+7200+3900= 22176.