Semi-major axis vs. apogee and perigee of orbit This article on Wikipedia about Sedna states that its aphelion is 937 AU, its perihelion is 76 AU and its semi-major axis is 484 AU.
I would have expected the SMA to be half of the sum of the aphelion and perihelion. Is the article in error, or is there a difference that I'm not aware of?
I know apoapsis and periapsis are sometimes given relative the surface of the object being orbited, see this thread, but the effect should be negligible in this case and would anyway work in the opposite direction. Anything else I'm not seeing?
That's Wikipedia for you. The linked page demonstrates that Wikipedia is not a reliable source. Here are some of the problems:

The values for aphelion, perihelion, semi-major axis, and eccentricity are inconsistent with one other.
The values for perihelion, semi-major axis, and eccentricity are unsourced.
The values for perihelion, semi-major axis, and eccentricity have far too much precision. Sedna was discovered 17 years ago, corresponding to about 0.15% of Sedna's orbital period. That's far too short of an arc length to justify five or six places of accuracy.
The value for semi-major axis is inconsistent with other sources.
The value for aphelion is poorly conducted original research. Footnote 5 (the reference for the aphelion value) states that the source is from osculating orbital elements about the solar system barycenter as retrieved from JPL Horizons.
There are two key things wrong with this. One is the use of osculating elements, which can be deceiving. The other is using orbital elements about the solar system barycenter rather than about the Sun. This is just wrong.