The Lower the Ampere Turn Spec the more sensitive the reed switch will be.
The first figure in the Spec is the operate spec. When two identical magnets approach the two reed switches the one with the lower operate Ampere Turns is more sensitive to the magnetic field and will close first.
The second number is the release Ampere Turns. When two identical magnets move away from the two-closed reed switched the one with the higher release Ampere Turns is less sensitive and will open first.
So if we were to look at two similar parts HE506-ND (AT rating 15-25) and part HE532-ND (AT Rating 30-35), and assuming we are using the same magnet the HE506-ND would be the first to sense the magnet as it approches and close. The HE532-ND would be less sensitive and would be the first to open as the magnet was pulled away from the switch.