Indeed you are right. If you could draw a process-flow chart showing the numerous steps taken to produce a shot of espresso you would be amazed at how many micro-processes take place. Its not rocket science....but there are a number of places in the process where lack of calibration...or a failure to repeat a process the same way that you have done 1000 times before can change the final taste and appearance of the espresso. That process checklist includes....water temperature, machine pressure, the grind of the bean, the amount of grind dispensed from the dispenser, the tapper you use, the ammount of pressure you apply when tapping, the flatness of the tapper in the basket.... and this is even before you pull the shot!