Quick speculation: It could simply come down to being better fed over time. There is evidence of negative multi-generational effects on descendants from malnour

Quick speculation: It could simply come down to being better fed over time. There is evidence of negative multi-generational effects on descendants from malnourishment suffered by progenitors. If nobles had better access to food (which makes sense, given power differences), that could propagate down the generations (and promote increased reproduction) not due to a stronger gene pool, but rather through improved gene expression and general health.