The temperature difference between the Tilt Hydrometer and the temperature probe in the GF30 Conical Fermenter can be caused by the Tilt's position in the fermenter.
The Tilt has a natural tendency to move towards the edge of the fermenter, this is called the Cheerios effect. When the GC4 is pumping glycol, the Tilt is touching the cooling jacket so, this will give a reading that is below the actual beer temperature.
Stratification within fermentation vessels is a known phenomenon, and this is always worse at the surface of the wort as you have much more thermal conductivity between the ambient air outside the fermenter, through the stainless lid which is not insulated and the headspace of the fermenter. This is then impacted further by the exothermic processes of the yeast and where they are active within the fermentation vessel.
This activity of the yeast helps to create thermal convection which to a large extent helps to regulate the temperature of the fermenter but this is not perfect and if there is no activity then there is no convection thus the fermenter is more susceptible to stratification. hence why most home brewing and commercial tank record and control the temperature from about the middle of the minimum batch volume not from the top of the liquid level.
The temperature difference can also occur due to inversion. There is convection occurring due to fermentation in the Conical; cold liquid sinks and warm liquid floats. The Conical Fermenter temperature probe is near the base, while the Tilt is placed at the top. When the beer is cold, the convection slows down and is no longer able to transfer the temperature as it does when it's warmer. So actually, both temperature readings are likely correct, it's just the probe is at the other end of the fermenter and further away from the cooling sleeve.
This is why we've set a minimum limit on the temperature setting of the conical - although the glycol can go down to -5°C, and the temperature in the beer next to the jacket could even freeze in extreme circumstances. There will be little uniformity throughout the fermenter, so it's quite difficult to gauge as it gets towards the limits of the unit.
As a result, the design of the fermenter takes this into account with the placement of the temperature probe to give the best average across the system.