Displayed temperature is not correct K8400

Hi guys,

I have a problem with my K8400. I changed the hotend to a E3D V6. Used the orginal heater cartridge. Changed the temp thermistor to the one from E3d V6 (ATC Semitec 104GT-2).

My problem is now this:
When i want to print PLA at 205degrees celcius, the actual temp (meassured with a thermocouple on a fluke meter) is around 187,5 degrees celcius. I meassurd this directly next the thermistor.

I tried to change the settings in the configuration.h file. The temp senos was allready at number 5, the correct one.
5 is 100K thermistor - ATC Semitec 104GT-2 (Used in ParCan & J-Head) (4.7k pullup)

I have already installed a new sensor, the same type, but this one has the same temperature difference.

So, i hope one of you guys can help me out with this problem!

Best regards, Jacco