The empirical data only tells you that your mass result is variable. It's your theorising that suggest as a hypothesis that the lake's current volume is perturbing your measurement - which you then show to be consistent with the rest of your theoretical framework by experiment.

I wonder if the bounds are such that someone walking along the corridor will affect the measurement ("yo momma so fat she perturbs the expected measured mass of the Z-boson!"). What's the relative gravitational attraction of, say, Jupiter versus a nearby train.

A great insight.