I never saw the definitive source on this, but around the time when the unionization vote was happening, people claiming experience in the logistics industry described Amazon as a place with low starting wages, but aggressive bonus structure for high performers. They also offered health benefits on day 1, which is unheard of in logistics.
Other warehouses in the area might have different compensation schemes, and people would generally gravitate to the one that suited their work style better.
The unionization vote seemed like it corroborated this thesis - it wasn’t even 52/48, but something definitive, like 70/30 against the union.
Perhaps someone is aware of the data that incorporates take-home compensation with benefits and all, not the base rates advertised in job ads.