Currently, players need to accrue an average of 3.1 plate appearances for each game their team plays in order to qualify for the batting title. An exception to this qualification rule is that, if a player falls short of the 3.1 plate appearances per game but would still have the highest batting average even if enough hitless at bats were added to reach the 3.1 mark, the player still wins the batting championship. The latest example of this exception being employed occurred in 1996 when Tony Gwynn had 159 hits in 451 at bats for a .353 batting average but had only 498 plate appearances, four short of the 502 necessary. Since four hitless at bats would have lowered his batting average to .349 but .349 was still better than anyone else in the league, Gwynn was named the National League batting champion. 
The 1902 AL batting championship was awarded to Ed Delahanty at the time, and the Baseball Hall of Fame's statistics still credit Delahanty as the champion over Lajoie. The Hall of Fame lists Lajoie's 1902 stats as being 129 H / 352 AB / .368 BA, while Baseball-reference.com lists them as 133 H / 352 AB / .378 BA, which would place Lajoie above Delahanty.
Additionally, Lajoie had only 371 plate appearances to Delahanty's 535 or 536 (depending on which source you use), so there may also have been an issue of eligibility — Lajoie's 371 plate appearances falls well below the 424 that would be required under the current 3.1 plate appearances per team game standard of eligibility, and adding 53 hitless at bats would drop his average to .314.
This discrepancy is somewhat significant, as Delahanty is regarded as the only player to win a batting title in both leagues (.410, 1899 Phillies), as of the end of the 2005 season.
|1882||# Pete Browning||Louisville Eclipse||.378|
|1883||# Ed Swartwood||Pittsburgh Alleghenies||.357|
|1884||# Dave Orr||New York Metropolitans||.354|
|1885||# Pete Browning||Louisville Colonels||.362|
|1886||# Guy Hecker||Louisville Colonels||.341|
|1887||# Tip O'Neill||St. Louis Browns||.435|
|1888||# Tip O'Neill||St. Louis Browns||.335|
|1889||# Tommy Tucker||Baltimore Orioles||.372|
|1890||# Jimmy Wolf||Louisville Colonels||.363|
|1891||# Dan Brouthers||Boston Reds||.350|
|1914||# Benny Kauff||Indianapolis Hoosiers||.370|
|1915||# Benny Kauff||Brooklyn Tip-Tops||.342|
|1890||# Pete Browning||Cleveland Infants||.373|
|1884||# Fred Dunlap||St. Louis Maroons||.412|
|1871||# Levi Meyerle||Philadelphia Athletics||.492|
|1872||# Ross Barnes||Boston Red Stockings||.432|
|1873||# Ross Barnes||Boston Red Stockings||.425|
|1874||# Levi Meyerle||Chicago White Stockings||.394|
|1875||# Deacon White||Boston Red Stockings||.367|