Hall sensor testing

For a personal project, I’d like to use Hall sensors to measure water level. But how do you pick a Hall Sensor when they all claim the same characteristics?

​For this test, I ordered 3 different Hall Sensors in SOT-23-3 package. All 3 are omnipolar, so they will react to both North and South magnets. They all claim to have a Operation Point of 40 gauss, noted as “Bops”. The Max Bops value is a little different, but in theory, they should all trigger at the same distance. Guess what? They don’t:

Hall Sensor Model	Trig with 8x3mm round Neodym	Trig with 60x15mm square Ferrite
CC6201ST	11 mm	21 mm
AH180-WG-7	14 mm	25 mm
APX8132AI-TRG	15 mm	33 mm

I measured this quite roughly using calipers held up next to the magnets, but the deviations between the sensors are quite significant. So - why the difference? How can they all claim the same capabilities in the datasheet and still be this different?