Because it consists of two half steps. If you look at a keyboard, you can see that there is a note in between these two notes.
Explanation:
You can count any number of whole steps or half steps between notes; just remember to count all sharp or flat notes (the black keys on a keyboard) as well as all the natural notes (the white keys) that are in between. Example 4.2. The interval between C and the F above it is 5 half steps, or two and a half steps.
I hope this somewhat helped. Try and not to focus so much on the white key notes on the keyboard ^^
