You can find the reasons in one or more of the 4M’s of proper espresso making, made famous by the Italians:
Miscela: The coffee-blend; it covers the selection and blending of the beans as well as proper roasting.
Macinazione: The grinder; proper grinding of the given blend and its usage when its still fresh.
Macchina: The espresso-machine; usage of the given grounds with a proper espresso-machine.
Mano: Skill of the barista; usage of all above with skill.
Assuming that the beans are same, your reasons will lie in one or more of the other three. This is equally applicable to other brew methods too.