John Wardigo posted a great write-up concerning the confusion over different Antifreezes ... thought it was worth archiving and sharing on CinciTDI:

A Little Chemistry

To begin with all antifreeze is wither ethylene glycol or propylene glycol. The only significant difference between the two is toxicity. Ethylene glycol is very toxic and can result in death... this is why animal rights activists pushed for the use of proplyene glycol in antifreeze. Propylene glycol (PG) is used in things such as toothpase, cosmetics, food, solvents for phameceuticals and food colors, cooling agent for beer and wine fernentation tanks and in the current H1N1 scare is used in hand sanitizers.

Now the big issue is the additives. Many US and Japanese auto manufacturers use a phosphate inhibitor. It is believed the phosphate inhibitor by protecting aluminum engine components and protects ferrous metals (iron). It also acts as a buffer to maintain the mixture alkaline. A buffer will absorb acidity produced in the engine either from the break down of oil or unburnt fuel in the crankcase or from blow y in the engine. By preventing the buildup of acidity you prevent the damage of engine parts. I am sure many are thinking ... isnt that was the oil is for... yes, but you don't want to mix the oil and water...

This is where the significant difference comes in. European manufacturers believe the benefits are achieved with the use of non phosphate based products. Their thought is the phosphate based inhibitors have the potential for the drop out of solids, especially when mixed with hard water. The solids will collect on the cooling system walls and will develop the scale on the side of the cooling system. This would reduce the cooling capabilities fro the engine and would generate hot spots on the engine and reduce the life of the system. These concerns with hard water are because water in Europe is much harder than water in the US.

So the question as to why they are different... well the main base of cooling the engine are essentially the same, what i see as the primary difference is the additive chemicals for rust and corrosion inhibitors. When push comes to shove and something happens to the engine, the manufacturer can do a chemical analysis and put the blame on you for not using the proper antifreeze. Me personally i know about the whole hard water issue and try to keep these dissolved solids out of my system buy purchasing distilled water from the grocery store. cost me about 3-4 bucks for 100% "high quality H20" for my car.