Colors in computer are based on Red-Green-Blue. But deskjet color printers use Red-Yellow-Blue inks.

Why are they different ?

it's actually RGB and CMYK(Cyan, Magenta, Yellow, Black). This site has a detail description http://www.harrywaldman.com/June%2014%2004%20Pbol.html
and
http://www.sparklenet.com/1_Main_Street/Ar...CMYK_vs_RGB.htm

There are also several other color picking variations. One very useful one is HSV (Hue, Saturation, Value) where Hue picks the color, Saturation determines how deep the color is, and Value determines how bright the color is. This can be a little misleading though, because yellow is a naturally more bright color than, say, red. One of the reasons for using RGB in screens (and thus computers) is that the human eye only has red, green, and blue receptors. The other colors are mixes of those three, so it is natural to emit light from devices in those colors. Since paper absorbs light more than reflects (emits) it is pigment based, so the primary pigments, Cyan, Magenta, Yellow are used, where each pigment absorbs one of the primary light colors, Red, Green, Blue respectively.

~Viz

I had never really thought about that, which is odd since I'm usually the first to get curious about inconsistencies like that and look it up haha. Interesting to read about... it's funny that more people aren't thrown by that when going to buy ink and wondering why such a weird combo of colors was chosen (CYMK) haha

Actually, it has to do with transmissive and reflective sources. Your monitor is a transmissive light source - light is being transmitted out of your monitor. Paper is a reflective source, light reflects off the paper and inkpigments into your eyes.

Rather simplified explanation, but they are two totally different light sources.