Based on what storms this size usually create, they gave estimates. I mostly read geeky real weather sites, and I saw much larger estimates than what happened. Understatement based on minimums seems more irresponsible, so I know what I'd rather believe until the reality proves me wrong (or right).
This is one of the few mainstream articles I've read, and I'm not a meteorologist, but it enlightened me on a few points.
https://www.nytimes.com/2017/09/11/us/storm-surge-irma-flooding.html