I am F('d) on this Probability Density Function understand, everything

Elliotbay

Schweser Secret Sauce:

Prob(x) = x/15 for x =1,2,3,4,5

F(3) or Prob (x < or equal to 3) is 1/15 + 2/15 + 3/15 = 1/3

This is simply the sume of the probabilities of 1, 2, and 3.

(I understand everything above, here is what I don't understand

Note that Prob (x = 3, 4) can be calculated as
F(4) - F(2) = 10/15 - 3/15 = 7/15

Why does he subtract F(2) and not F(3)?