The words rosary, rosery sound the same but have different meanings and spellings. Why do rosary, rosery sound the same even though they are completely different words?
The answer is simple: rosary, rosery are homophones of the English language.
Roman Catholic Church A form of devotion to the Virgin Mary, chiefly consisting of three sets of five decades each of the Hail Mary, each decade preceded by the Lord's Prayer and ending with a doxology.
Roman Catholic Church One of these sets of decades.
Roman Catholic Church A string of beads of 5 or 15 decades on which these prayers are counted.
Similar beads used by other religious groups.
A place where roses are cultivated; a nursery of roses.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, 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").