Simple - prior to issue 94, the book was called The X-Men. It was renamed X-Men when it came back from hiatus after Giant-Sized X-Men 1, though they were referred to as the Uncanny X-Men in the introduction text at the start of each issue. The title changed to The Uncanny X-Men with issue 114.
As a result, Marvel Masterworks collections for 1-66 (because 67-93 were reprints) were called The X-Men volumes 1-6, while the collections for Giant Size X-Men 1, X-Men 94-113, and Uncanny 114-140 were Uncanny X-Men 1-5.
The naming convention has more or less continued with subsequent collections.