@Flumble: Ugh, yeah, I noticed that, but the image doesn't appear to be on Wikipedia at all anymore. And the current response-curve image I could find doesn't have the little uptick in red response out past blue, which is key to the whole damn thing. Ughhhhhhhh.
Why does the digital image of the rainbow have extra red on the blue edge? (Not why our cones respond as though it did; why does the digital file itself really have redder pixels there?)
As Pfhorrest said, it's because that's how you make purple on screens. We don't have purple subpixels, just red, blue, and green; due to the quirk of our eyes explained in my blog post, blue + red happens to resemble spectral purple. So, there's a lot of red in the colors on the red edge (duh), and on the purple edge, but little to none in the middle where green/blue dominate.
Soooo... in short... we see a bit of red because red is what we see.
Heh, not quite. I'll be a little more specific:
We only see colors distinctly due to our three types of cones responding differently. (In low light, where only our single variety of rods is active, we only see things as light/dark, there's no distinct colors beyond what our brain fills in from our knowledge of what colors things *should* be.)
At spectral blue, we have a high blue response, and roughly zero red and green response. If the response curves were just as shown in Wikipedia, then going further to spectral purple would just give us a medium blue response, and still roughly zero red and green response, so it would just look like a dark blue.
But there's a little uptick in the red response curve out past blue's frequency, so when you hit spectral purple, you actually get a medium blue response, a low red response, and a ~zero green response. This is a unique response combination, so it gives us a unique visual color - purple.
And, because of this, we can mimic spectral purple by just combining blue light with a little bit of red light, producing a similar response curve and letting us see a non-spectral purple. Thus, the red component of the pixels on the purple side is raised up.
(Only a small portion of the space between blue and red on the color wheel is actually taken up by spectral purple or its non-spectral look-alikes. Most of the space has too much red to actually mimic a spectral color; the entire "magenta" color area is 100% non-spectral, a color of light that does not exist as a single wavelength in reality. (And the "purple" in that spectrum is mostly magenta.) This isn't too weird - white light is exactly the same. In fact, magenta and white are the only non-spectral colors we can see, without hacking our biology more, like fatiguing your red sensors by looking at a red square then looking a green square to see "super-green".)