The words overbilled, overbuild sound the same but have different meanings and spellings. Why do overbilled, overbuild sound the same even though they are completely different words?
The answer is simple: overbilled, overbuild are homophones of the English language.
Simple past tense and past participle of overbill.
To build over or on top of.
To construct more buildings in (an area) than necessary.
To build with excessive size or elaboration.
To construct more homes, office buildings, or commercial complexes than necessary in an area.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, 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").