Categorization
From Brickwiki
Categorization of LEGO sets is highly problematical as it is Adult Fans of LEGO (AFOLs), collectors and sellers who are concerned with finding the right theme or subtheme in which to place every single product LEGO sells or has ever sold.
The company itself has not shown much concern for categorization other than ensuring current product lines are visually distinct. This can lead to confusion especially with large themes such as Town. Personal opinions often interfere with effective categorization as someone who is a trainhead may not agree with someone who claims trains is a subtheme of Town.