Seems pretty obvious that it must be a mistake. There's no way the 24" would be cheaper than the 20", and besides that, the model number 2005FP.
Edit: Adding that choice to your cart, I see:
Monitors Dell 24 inch UltraSharp 2005FP Widescreen, adjustable stand, VGA/DVI 2405FPW [320-4188] 5
Since it does mention 2405FPW, maybe it is real. If so, the price must be wrong because it's like $300 cheaper than the 2005FP
Edit: Adding that choice to your cart, I see:
Monitors Dell 24 inch UltraSharp 2005FP Widescreen, adjustable stand, VGA/DVI 2405FPW [320-4188] 5
Since it does mention 2405FPW, maybe it is real. If so, the price must be wrong because it's like $300 cheaper than the 2005FP