I'm still learning myself, but it's potentially because the machines are already warmed up and the grinder is "dialled in" for the beans on offer. One very important aspect (among other things such as temperature, pressure etc) with espresso is the grind size, which will need adjusting between different bean types and roasts. Everything else being equal, a grind that's too coarse will let the water through too quickly and give you an under-extracted "sour" tasting coffee, whereas too fine and the water moves too slowly and you end up overextracting (and/or choking the machine) and it gets very astringent and bitter tasting; not pleasant at all.

Alternatively, they could be using a pressurised portafilter, which give much more consistent results, but do take away some control and limit the end result. If it's a high end machine though it's probably an unpressured filter basket.

Edit: Also the roast makes a big difference to how difficult it is to pull a good tasting espresso. Many restaurants/ chains use fairly dark roasted beans which are generally a lot more forgiving than lighter roasts. At least that's been my experience.