“In fiscal 2018, 70% of approved H-1B petitions were for workers 30 years of age and older—a significant indicator that those workers already possess at least six to eight years of experience. Further, H-1B workers’ educational levels, which are an important determinant of skills, indicate they should be filling higher-skilled positions. In fact, 63% of all H-1B workers held an advanced degree (master’s, professional, or doctorate degree),32 meaning one could reasonably conclude that a majority of H-1B workers have the educational attainment and/or years of experience to fill positions at wage levels 3 and 4. These data suggest it is likely that H-1B employers are underpaying workers relative to their skill levels.”

https://www.epi.org/publication/h-1b-visas-and-prevailing-wa...

Anecdotally, I have seen the h1b under leveling happen multiple times. But not sure it’s common enough to skew the data but it does stand out when it happens because you have a great engineer with 10 years experience and you find out they are an SDE1. For every one of those there are probably 10 that are correctly leveled or over leveled.