Your Dell computers are probably using much cheaper, 6 bit/channel (18bit) or 5/6/5bit (16bit) screens, and are likely not LED LCD screens but standard CCFL LCD screens. Browsers that support ICM will generally render photos with more accurate color than browsers that do not when an ICM profile is present in an image. Additionally, Safari, which I assume you are using on the Mac, supports ICM. They are much higher quality displays than your average LCD screen, and specifically designed to output high quality, rich, saturated graphics.
These are full 8 bit/channel (24bit) screens and offer a full 178° viewing angle. The 27' LED mac displays are 'full gamut' displays, ones that cover around 98% of the Adobe RGB gamut.
