Going from a medical resident to doctor moves you from one income strata to another. It's not a sign of income mobility.

There is some income mobility in the US. However people generally make more money by changing what they do. Which generally means someone else ends up in the old job. AKA if a janitor goes to night school and becomes a mechanic that does not change what janitors makes.

Looking at the income of individual segments doesn't tell you anything about mobility.

Income could remain static, but people could move from higher to lower segments and vice versa.

The normal way to compare mobility is to look at someones income percentile at age X vs parents income percentile at age X. After all most people's income is ~zero as a young teen.