The words serge, surge sound the same but have different meanings and spellings. Why do serge, surge sound the same even though they are completely different words?
The answer is simple: serge, surge are homophones of the English language.
A twilled cloth of worsted or worsted and wool, often used for suits.
To overcast (the raw edges of a fabric) to prevent unraveling.
To rise and move in a billowing or swelling manner.
To roll or be tossed about on waves, as a boat.
To move like advancing waves: The fans surged forward to see the movie star.
To increase suddenly: As favorable reviews came out, interest in the software surged.
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.