The gaokao is a national exam only in name. Papers differ by province (though there is significant convergence around a small number of national paper sets), and admission quotas are set at the provincial level.
Henan sends more than a million candidates per year and receives a comparatively small share of top-university seats. The score required to enter, say, Peking University from Henan is meaningfully higher than the score required from Beijing.
This is not a secret and it is not accidental. Universities administered by a city or province tend to enroll more students from that region, and the biggest universities are concentrated in a handful of provinces.
The upshot is that your gaokao score is only meaningful in comparison to other students in your province. National ranking lists exist but are not what universities actually use.
One consequence: transferring your household registration to move for the gaokao is legally and practically difficult, and the state actively discourages 'high-migration' (高考移民).