A fence is mostly a long run of the same thing, so it prices close to its length. A deck is not — the boards are the easy part, and the rails, spindles, steps and skirting are where the hours go. A small deck with heavy railing detail can take longer than a larger plain one.
Condition then moves it either way. Bare new wood is straightforward. A deck with a failing finish on it needs that dealt with first, and that is a real part of the scope rather than a footnote to it.