That's the standard fishing mortality definition that is in the fisheries biology, where it's basically on a logarithmic scale. It's not the same as percentages. For example, a fishing mortality, or F, of 0.2 is roughly 18%, and the difference increases as you go higher up. You can fish at an F of 1.0, and that is not 100% of the stock. You can even fish at 1.2.
This kind of confusion, or the technicality of that term, was one of the main reasons it was decided to go with harvest rate in Iceland, which is this 20% of some reference biomass. It's much easier for people to understand that as a technical fishing term.