The chillers have a 1 degree difference between the on and off switching points, this is to prevent the compressor cycling too often and damaging the unit.

For example if you set the chiller to 26 degrees it will trigger at 27 degrees then chill back down to 26 degrees.

The real world result isn’t usually a swing in tank of one degree as the chiller reads the internal temperature of the water running through it rather than the tank temp, the actual swing inside the tank will actually be much smaller.