I have measured differences of about 30C degrees between different build plate areas. The most cold is the left part where the cables enter the plate, way beyond the most hottest point which is located on the ‘center bottom’ part.
When the sensor feels 85C there are above 90C on the hottest point.
The distribution of heat is not uniform, it follows, more or less, the shape of the resistor behind the plate.
The following pictures show the heat distribution when bed is heated at 85C.
The first picture is the whole bed with a center temperature of about 84.8C which is pretty near the requested 85C.However there are hotter areas around, up to 93.4 (the white areas).
The second image is a detail of the bed center; temperature goes from 80.9C to 91.3C.