I have somewhat the same experience, although Im finally getting to where I can get very similar tastes more often than not--especially if I work at it.
As the different chemicals extract at different times and rates throughout the pour it is going to take an *extremely* consistent production to duplicate the taste of the previous cup. It requires the same beans, the same quantity of beans, the same dose and tamp and *exactly* the same amount of coffee expressed in the same time by the same temperature of water. Then the milk has to be frothed to exactly the same temperature and volume.
Ive come to the conclusion that *almost identical* does not do for identical taste.