I've worked for few different companies where some development groups were producing negative productivity, and others were producing positive productivity. Consider that one "profitable product ruined", and one "unprofitable product made profitable", or equivalent metrics.

What's the productivity ratio between -1 and +1? Is the first team "less productive" than the second team? Certainly. Is the second team "twice as productive" as the first team? I think the answer is "infinity", so "over twice" is a good summary for people who intuitively distrust "10x" as a number.