To prevent the tags from wandering. AO3 will associate disambiguated tags with other related fandoms. I took a look at the tagsets from previous years and the tags will wander back and forth if using 'Fallen London' and 'Sunless Sea' as the disambiguation. As I'm done fighting AO3 with trying to figure out variations that won't wander, I now use silly disambiguations that either have a joke or share the same initials. In this case it is sharing the same initials.