You are missing the complete musical alphabet including the accidentals; the sharps and flats in between some of the letters.

This is all 12 possible musical notes (the chromatic scale):

A - A# - B - C - C# - D - D# - E - F - F# - G - G#

The distance between each of those is a half step (1 fret distance). So, some of the musical letters A through G are a whole step (2 half steps) or 2 frets apart, but some are only a half step apart (B and C, E and F). This means any time you include the B & C or E & F you have to be careful to account for the accidentals.

The musical distance of a fifth is 7 half-steps, or 3 and 1/2 whole steps. And sometimes you can just count letters. For example: what is the 5th of C?

You can rely on just counting letters in this case.

C - D - E - F - G

It works because the E to F half-step doesn't interfere.

C - C# - D - D# - E - F - F# - G

That's 7 half-steps from C to G.

But if you try to figure out the 5th of B you run into trouble because you are landing on one of the exceptions.

B - C - D - E - F

Looks like F right? Not so fast! :) You have count the actual musical intervals to make sure you have 7 half-steps.

B - C - C# - D - D# - E - F - F#

The lesson here is that you need to be aware of the complete musical alphabet system including all the sharps and flats. And that you need to be aware of what the actual musical interval distances are, not just counting the shorthand of counting letters.

Hope this helps!