The battery reads 81% for one reason only: No (sane) laptop will ever fully discharge to zero when it reads 0%. By default, there is a 4% reservoir on windows, and it adjusts behind-the-scenes. 100% is 100%, but 0% is 4% (saves your battery).
Here’s a graph that explains it (X-axis is actual charge, y-axis is displayed)
Because it’s 80.8%, it rounds up over 80.5%