Pixel density and resolution are not one and the same.
iPad 3: 2048-by-1536-pixel resolution at 264 pixels per inch (ppi)
rMBP: 2880-by-1800 resolution at 220 pixels per inch
So even though the iPad has a significantly higher pixel density, it's only pushing 3,145,728 pixels, whereas the rMBP is pushing 5,184,000 pixels. That's 2,038,272 more pixels for the rMBP.
Apple doesn't use the term "Retina Display" to specify a given pixel density. Rather, it refers to a pixel density at which one cannot distinguish individual pixels from the normal viewing range of the device. The shorter the distance between your eyes and the device screen, the higher the pixel density has to be in order to not see individual pixels.