The words primer, primmer sound the same but have different meanings and spellings. Why do primer, primmer sound the same even though they are completely different words?
The answer is simple: primer, primmer are homophones of the English language.
the first or preliminary coat of paint or size applied to a surface
an introductory textbook
any igniter that is used to initiate the burning of a propellant
A <xref>device</xref> used to circulate <xref>gasoline</xref> into the ignition chamber of an engine.
<xref>comparative</xref> form of <xref>prim</xref>: more <xref>prim</xref>
Definitions from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved., 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.