The words precisian, precision sound the same but have different meanings and spellings. Why do precisian, precision sound the same even though they are completely different words?
The answer is simple: precisian, precision are homophones of the English language.
One who is strict and precise in adherence to established rules, forms, or standards, especially with regard to religious observance or moral behavior.
The state or quality of being precise; exactness.
The ability of a measurement to be consistently reproduced.
The number of significant digits to which a value has been reliably measured.
Used or intended for accurate or exact measurement: a precision tool.
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").