If you poured the second beer into a different glass, the glass temperature may have effected the reading. Pour the second beer into the same glass. This should be the actual beer temperature. A bottle of water on top of a keg may be in the direct path of the evaporator fan and be colder than the beer.
A reading from a dispensed beer is always more accurate.
Scott Zuhse, Instructor
Micro Matic Dispense Institute