Are Psilocybe cubensis strains really that different from one another?

Yes, and no. Though strains within the same species have an almost identical physical appearance when it comes to their spores, there are subtle nuances that make them each unique. Another obvious example is derived from the fruits themselves, which have a large amount of variance in terms of their physical appearance.

It is the same concept as any other organism of the same species. Think of humans, we each appear the same from the perspective of a sperm cell, but once we’ve fully formed we begin to take on unique physical and chemical characteristics.