I picked up the guitar a about two years ago but I know almost nothing about music theory (outside of what I got in elementary school) so i have just been reallying on tab and my ear to get me along. I've found the lessons on notes very helpful but i was hoping for more of an explanation.

Specifically I was wondering why not all strings play sharp notes when you press down on the first fret? So when I play the high E, B, and low E while pressing down on the first fret I get a F,C, and F. Conversely when I play the G, D, and A strings while pressing the first fret I get a G#, D#, and A#.

If each fret is a half step up on the musical staff then why isn't the pattern of sharp notes consistent throughout the first fret?