Here's why:
* The double "m" represents a single sound: The "mm" in "accommodate" is a single consonant sound, similar to the "m" in "mat." It's not like "hammer" where the "mm" represents two separate "m" sounds.
* The "mm" is a digraph: This means it's two letters representing one sound.
The word "accommodate" is pronounced with one "m" sound, even though it is spelled with two.