The PE-2 unit has a 6"1 D:S ratio. At 6 inches away you are measuring a one inch spot. At a foot away you are measuring a 2 inch spot. At two feet away a four inch spot. That information is included in the instructions.

Within a four inch area (or certainly a 12 inch area) you may see a number of different temperatures, especially if your heat tape or bulb is heating smaller area. You are reading an aggregated average of the entire spot.

I typically use the PE-2 unit at a foot away or less, for reptile setup temps.