The words phasing, fazing sound the same but have different meanings and spellings. Why do phasing, fazing sound the same even though they are completely different words?
The answer is simple: phasing, fazing are homophones of the English language.
Present participle of faze.
Present participle of phase.
Movement through phases; arrangement of a sequence or cycle.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License 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").