Complex sentence: The reason is that etymologically (also in accents without the horse--hoarse merger), the words have different vowels, "forty" containing a contraction in the same way that "fifty" contains a contraction of "five".
Split into simpler sentences: The reason is that etymologically (also in accents without the horse--hoarse merger) the words have different vowels. "Forty" containing a contraction in the same way that "fifty" contains a contraction of "five".