The answer is simple, if the cost was $300 more they wouldn't sell any.
In inches or metric or Hittite Cubits it is the same size - large. That is probably why it is called the Terrazzo 64 in. LARGE BBQ Grill Cover although it is also possible they misspelled the word "Largo" or even "Barge". The grill I own has dimensions of 64 inches wide / 48 inches tall / 24 inches deep. This cover fits snugly and has an elastic opening so it's dimensions are approximately the same. Please note if you have the same grill pictured above with one wheel and a post sticking out the bottom it will also fit.