https://www.mlb.com/news/faq-negro-l...-league-record
They only have 75% of the data.
"However, because of the inconsistencies of Negro Leagues team schedules (or the available data), the minimum qualifier for each league and season is based upon the average number of games played by each team, multiplied by 3.1 plate appearances for hitters and one inning for pitchers. Those values are subject to change as more data is discovered.
As for career leaderboards, the current standard for career MLB leaders is 5,000 at-bats and 2,000 innings pitched, which roughly equates to 10 full qualifying seasons (5,020 at-bats and 1,620 innings). Therefore, for Negro Leagues players, this standard has been set at 1,800 at-bats and 600 innings -- roughly the equivalent of 10 seasons’ worth of 60-game seasons."