The power needed is determined by the product of the total drag force (incl. aerodynamic, rolling, gravitational, and mechanical losses) and your speed. But there could be small changes in the drag force depending on the gear ratio you choose - aero drag could change with cadence as your legs move faster or your body positioning changes slightly, mechanical losses can change with a different chainline and chain tension. However, I'd expect the changes to be pretty small.
Another cause for what you saw would be inaccuracies in the power meter. It has sensors to measure your pressure on the crank and therefore the effective torque and also determines your cadence in order to estimate your power input to the bike. If those sensors aren't perfectly linear then you'd get a different reading in a lower gear with decreased torque but higher cadence even if the actual power didn't change.