The reason is that the Berks & Hants Railway Company, under the powers of which the line was built, also included the Reading-Basingstoke section. The company was an offshoot of the G.W.R., which absorbed it before any section was opened.
dcterms:bibliographicCitation
1945 September and October, “Notes and News: The "Berks & Hants" Line”, in Railway Magazine, page 299: