A few factors could explain the differences --
Lighting -- since Taylor's sunbursts tend to be a little more delicate and less opaque than say, a Gibson, they seem to be more susceptible to change depending on the lighting. For example, sometimes Taylor's tobacco sunburst looks darker like Gibson, and other times it has a brownish or even reddish tint in the dark areas when under certain lighting conditions (especially casual flash photography).
Age -- not necessarily that the sunburst has aged (although that could be a factor in some cases), but that Taylor's sunbursts have changed over the years (I personally think they just keep getting better) and I think they're currently darker and more well-defined like the photo Left Coaster posted -- likely because they've introduced several different edgeburst type finishes for lighter bursts?
Variation -- when I took the Taylor tour a couple years ago, they were still doing the sunbursts by hand -- which means you're going to have variation from guitar to guitar, day to day, even if it's the same person doing every single one.
Just a few guesses -- don't want to get into another sunburst debate
