Difficult to answer without stating the obvious! Logic dictates that either (1) it's an unusually long distance (which it isn't) or (2) the train travels unusually slowly between these stops (possibly due to speed restrictions). I've never noticed it myself, I must say. If you really want to experience the rare thrill of a long tube journey between stops - and let's face it, who doesn't? - I recommend the Central Line east of Liverpool Street.
Between these two stations is one of the three 'ghost' stations on the Piccadilly Line - it used to be called Brompton Road. The other two are Down Street (west of Green Park) and York Road (North of Kings X)