It's probably because of radioactive dust blown from the ground that has settled on the sign and somewhat irradiated the sign itself. When the Chernobyl disaster happened they dug and turned all of the soil in the area so it couldn't blow around so much.I’m guessing it jumps from 120 to 600+ because he points it at the metal sign?
The most radioactive soil will be underneath and it's probably why those Russian soldiers got ill. Some of that soil will contain some really nasty stuff from the explosion which takes thousands of years to decay. It's best left well alone...