Because most of the loss is not caused by handling.
A cryogenic vessel boils off continuously whether anybody opens it or not. That baseline is set by the static evaporation rate, measured with the vessel closed and undisturbed. Everything you do on top of that, every lid lift and every cane retrieval, adds heat the measurement deliberately excluded.
In a busy store the working losses are real but the standing losses usually dominate. So a laboratory that halves its sample handling often sees very little change in its nitrogen bill, and concludes wrongly that the figures are unreliable.
The practical consequence is that consumption is a vessel measurement rather than an activity measurement. Which is exactly why a rise at unchanged workload is diagnostic: with the activity held constant, the only variable left is the vessel.
