Arduino Sleep Mode Guide is an engineering topic related to battery design. It helps designers estimate values, avoid common mistakes and choose practical design parameters.
Real results differ because of tolerances, temperature, PCB layout, parasitics, cable losses, power supply behavior and measurement conditions.
Use formulas and calculators as a starting point, then verify with datasheets, simulations, prototypes and real measurements.
Runtime can be lower because of regulator losses, battery voltage sag, temperature, cutoff voltage, aging and peak current demand.
Average current is usually the most important value for runtime, especially in sleep-heavy IoT devices.