What factor causes rarity classifications for terrier breeds to be considered fluid or inconsistent?

Answer

The title shifts depending on which registry you consult, the year the data was collected, or the country surveyed

Rarity is not a binary state; it is heavily influenced by the specific dataset used, whether it is AKC registrations or international club data.

What factor causes rarity classifications for terrier breeds to be considered fluid or inconsistent?
DograrestbreedTerrier