The words valance, valence sound the same but have different meanings and spellings. Why do valance, valence sound the same even though they are completely different words?
The answer is simple: valance, valence are homophones of the English language.
An ornamental drapery hung across a top edge, as of a bed, table, or canopy.
A short drapery, decorative board, or metal strip mounted especially across the top of a window to conceal structural fixtures.
To supply with valances or a valance.
Chemistry The combining capacity of an atom or radical determined by the number of electrons that it will lose, add, or share when it reacts with other atoms.
Chemistry A positive or negative integer used to represent this capacity: The valences of copper are 1 and 2.
The number of binding sites of a molecule, such as an antibody or antigen.
The ability of a substance to interact with another or to produce an effect.
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.