understanding the datasheet of a normally closed / changeover reed switch, for powering Arduino project
The AT rating is given as a range because that's as good as this vendor is willing to specify.  It's always a balancing act.  Too sensitive and it could get triggered inadvertently.  Too insensitive and it takes too much power to switch it.
You should plan in your design to give it something ABOVE the maximum so that you are 100% sure it will switch when you want it to.  If you are in the middle, it's not guaranteed to switch.  It might, but then it might not.