Hysteresis is what controls the reaction of your temperature control.

Even though nominal value is set to 27.5C, because you have hysteresis at 1.0C (which is very high by the way), the heater OFF reaction will not be until 28.5C.
Set the hysteresis to something much smaller, 0.5.

In order for the chiller to turn ON at 28.4C, you will need to set the temperature settings to the following:

Nominal value: 27.5C
Hysteresis: 0.5C (Example)
Cooling difference: 0.48C

27.5 + (5/6 * 0.5) + 0.48 = 28.4C chiller activation point