The group sheds heat.
When measuring water temps without a portafilter, you are getting closer to boiler temps than shot temps.
When pulling a shot, the water fills the group and starts cooling, transferring it's heat to the group. As the shot progresses, the water temps drop. This is not a bad thing - it contributes to the overall lever profile.

A commercial group will shed heat faster, giving faster recovery times and needing some preheating.
A consumer group will overheat easily, leading some folks to cool the group with sponges and whatnot to get a faster recovery time between shots.