The 27" LED mac displays are "full gamut" displays, ones that cover around 98% of the Adobe RGB gamut. These are full 8 bit/channel (24bit) screens and offer a full 178° viewing angle. They are much higher quality displays than your average LCD screen, and specifically designed to output high quality, rich, saturated graphics. Additionally, Safari, which I assume you are using on the Mac, supports ICM. 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.

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. While these screens generally have higher refresh rates, and are great for gaming and movies, they do not reproduce color as accurately as diplays with higher bit depth and wider viewing angle. Contrast and color rendition on the cheaper Dell screens will generally cover the full sRGB gamut, but will fall quite short of the much richer and more saturated Adobe RGB gamut.

The Apple screens are middle ground, though. They use standard yellow phosphor/blue LED backlighting, which over the long term will often result in non-uniform color shift as the LED's age. CCFL backlit screens are less likely to encounter such color drift over the long term. For LED screens, full RGB LED will produce much cleaner, more consistent results over the long term. High end RGB LED screens designed for photo and graphics editing work will render the richest color (sometimes covering as much as 123% of the Adobe RGB gamut for maximum saturation) and purest white (blending RGB to form white more accurate than converting blue LED light into white light via a yellow phosphor, and is less subject to color drift over the long term.)