That's both a very short-term (years to days) and localized (in one specific sector of the economy) measurement. If you look at inter-generational mobility (as these studies do), and look at the level of mobility that you care about e.g. over a whole national economy (as these studies do), a son of such farmers who grows up to be in the same job as his father will be shown as moving around solely within the small slice of the class structure (a decile at most?) that makes up the farmers of the village. Even that much movement will only show up if his income is fairly consistently (averaged over at least a year) in a higher percentile than his father's. So yeah, I think relative mobility is a pretty good indicator.