Simple... two words: apparent wind

In light wind you keep building apparent wind until you are hauling and effectively "lit".

In heavy wind (25) you don't let the apparent wind build as much since you are ALREADY lit.

The efficiency of modern kites lets them build a LOT of apparent wind. And this is what makes new kites have such a huge wind range.

This is why I can ride a 9m in 17 these days, but a few years ago I needed a solid 25-30 to ride my 9m C kite.