Our expectations certainly play a role in how we experience any given strain, but this consistency is not insignificant. However, these expectations are sometimes over-generalized. Two types of cannabis compounds -- cannabinoids and terpenes -- hold most of the influence when it comes to effects. Cannabinoids like tetrahydrocannabinol (THC) and cannabidiol (CBD) are molecular structures with their own unique properties and medical benefits. Terpenes are the aromatic oils secreted in cannabis resin that modulate the effects of cannabinoids, and these too have their own set of effects. In this way, cannabis strains are the sum of smaller parts that may be passed on genetically. This helps explain the undeniable consistency in strain types, but also the variation.
Let’s take Blue Dream as an example. Due to its sativa-dominant genetics, we expect Blue Dream to make us feel uplifted and energized. Sometimes, however, you'll find a more indica-like phenotype, or a strain that expresses characteristics we associate with indicas, such as a shorter flowering time and bushier plants. How the plant is grown can also affect the strain's terpene and cannabinoid contents, and more or less of either compound type may give rise to different physical sensations.
As more research is conducted, our understanding of cannabis classification is bound to change and develop. Yet these categorizations still hold a great deal of importance, especially as breeders chase genetic lineages back to the earliest indica and sativa varieties.