The reason is because they just used two inks over top of each other, which is subtractive blending, instead of a magenta ink, which would be additive.

It's like the difference between paint and color spotlights.. mix paint and it goes black, mix light and it goes white. For the 3d glasses, you need to mix the colors like light does.

Although admittedly magenta is kind of lame looking, and most people don't look with 3d glasses, so maybe they were going for that.

So, there's one big problem with this method, the blue completely overlaps the red. If you look through one eye, you see a complete blue gem. If you look through the other eye, you see the red gem, but it has lots of holes in it.

Where the two gems overlap, instead of being blue, should be a mix (additive blend) of blue and red. Looks like some sort of magenta?