Why is battery runtime so much shorter than advertised?

Manufacturer runtime claims are almost always measured at the lowest speed. Owner threads on r/egopowerplus and r/Dewalt repeatedly point out that turbo or full-power use can cut runtime to 12–20 minutes on smaller batteries, while the advertised 60–90 minute figures only apply on low. Reviewers recommend buying a second battery or a higher-amp-hour pack if you have a large yard.