Real components generally contain internal resistance. This is not necessarily factored into a simulation.
Example: power supply, transformer, diodes, coils, etc.
Even capacitors have a certain amount of ESR, although we are not used to thinking it could be high enough to reduce performance.
I often add a few tenths of a volt in places, to bring greater realism to the simulation.
Another thing that might reduce power, is when a real transistor/mosfet does not turn on to the same degree as the simulated model.