[Gaz64]

Geof,

It's cooler at the back, because most engines run the coolant through the block from front to rear (cooled from the radiator), then through the heads from rear to front.
Although the bulk of the flow goes this way, there is small flow through steam holes etc between block and heads.

This is generally true for engines with the thermostat, the water pump and the hoses to and from the radiator at one end of the engine.

The hottest point is leaving the front the heads where the thermostat is located, which logically controls the temp of the engine.

Is a bit strange you see such a difference, but shows you must have a good radiator.

The block gets nowhere near as hot as the heads do, after all the combustion happens in the heads.

I'd be moving your temp sender to a similar location as your ECT sensor.

What does your ECT temp show in data?