The human controlling the chess computer, not a joke.
The time duration spent on inputting the moves will not be the same everytime, allowing the chess computer to do more thinking(pondering?), or for example calculating a different random number that determines the next move.

The only time the hardware may cause a difference in playing style, hmm... On Fidelity chesscomputers maybe? They have a 555 timer IC for generating IRQs, that chip will not give identical timing for each chesscomputer.