I haven’t grown a lot of “known” genetics to have a lot of first hand experience on the variety of expression shown within strains, however from my understanding its like familial differences.
for example, if you look at it like people, if you have a Mother and Father pair anything that is their child will be “white widow” and then anything that is “white widow” will have the genetic probability given by the combination of Mother and Father. so if Mom is blonde, and Dad is ginger, their children may have any color of hair from blonde, redish, bright red, and so on but in cannabis this is spread out over thousands of seeds, and people only have a couple kids on average. When you look at families with 10 or more kids, there is usually a varied showing of genetic traits throughout the siblings, some tall, some short, a few boys a few girls, and variance in eye and hair colors. One kid might have a heart defect, or that kid was 6ft tall at 12yo…
Conversely if you took a parental pair where both were record breaking athletes, mensa level genius’, and aesthetically “pleasing” their children would likely be naturally athletic, pretty “smart” and probably “cute” as well.
It’s the same idea in cannabis, if you have two plants that are purple, lemon scented, and produce large buds, their children will PROBABLY have large, lemony, purple buds, but not always.
When breeding plants however, you can back cross with other plants to add genetic stability by stacking the deck with a little more code from one of the parental plants. That way, when you “roll the genetic dice” you have already thrown your weighted dice in the cup. This way is more like having two athletic parents, instead of one athletic parent and one frail parent, the first example having a higher chance of producing athletic offspring, and the later having a wider variety of genetic traits in their lineage.