The on-die temperature sensor on the Atom is only designed to be accurate over the range of temperatures needed to control a fan and shut down the CPU if it overheats. It is very prone to "bottoming out" at low temperatures. Most likely, you are just reading the lowest temperature each on-die sensor can report, a temperature too low to activate the fan control or overheat logic and thus outside the on-die sensor's designed accuracy range.
Because each on-die temperature sensor is offset at the factory to ensure it activates thermal throttling at the correct temperature, the bottoming out temperature can be different on each CPU. While they're probably each internally reading the lowest temperature they can read, they are each adding a different calibration offset to this temperature and thus reporting a different measured temperature.
Some Intel CPUs have a more accurate thermistor temperature sensor bonded to the heat spreader. This is much more accurate for measuring CPU temperature than the on-die diode sensor (though it responds too slowly to be used as a thermal safety). I don't think the Atom's do though.