The words a, uh sound the same but have different meanings and spellings. Why do a, uh sound the same even though they are completely different words?
The answer is simple: a, uh are homophones of the English language.
The first letter of the modern English alphabet.
Any of the speech sounds represented by the letter a.
The first in a series.
Something shaped like the letter A.
Used to express hesitation or uncertainty.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").