I understand the English language can be confusing. At an early age, we are taught that adding an s to the end of words makes them plural. Sock becomes socks, caterpillar becomes caterpillars, and airplane becomes airplanes. However, every now and then you need to pluralize a word that already ends with an s, or one that ends with an x, z, ch, or sh. If you simply add an s to the word box, you end up with the word boxs, which, when spoken aloud, sounds like an utterance of a slurring, drunk person. This is why–complex as it may seem–some words are pluralized with an es rather than just an s.
Someone at Sonic missed the memo.