Calibration basic

It's because a dp transmitter does not measure flow, it measures delta-pressure in a pressure-loss type flowmeter. You're making some confusion: delta-pressure is pressure measurement, may be measured in psi, bar, mmH2O, mmHg, etc. kg/hr is mass flow, as is ppm. kg is unit for mass. Pressure transmitters do not measure mass. You may have meant 100 kg/sq cm or so.

The flowmeter then is designed to have a known dp for a given flow, and that's how we measure it. Mainly in compressive flows, it will vary with pressure and temperature, thus, you'll need a kind of flow computer to show you the flow directly. And there's your calibrated indication in flow.

Its in mmH2O because the less delta-p, the better for the process - less energy cost. But still need a good transmitter to assure consistent measurement. You could still measure it in bar or psi, but the measured value would be so small that would be hard to deal with.