The Oil temperature goes up and down as well though. Not as rapidly as the coolant to be fair. I suppose 'joe bloggs' may panic if they see coolant at 115 degrees not knowing any better.The reason for "90" being maybe 75C > 105C is that while the normal coolant temperature gauge is an analog device - it is being supplied with a signal from a digital source, so it will never move smoothly across its scale as it would if supplied by an analog more jump in steps - but as we tend to watch the road while driving we never notice that so this system works well enough as that gauge is only meant to be an indicator and not an accurate temperature gauge.
Another thing is to remember, if the car is filled with the correct coolant/water mix, even at ambient pressure, the boiling point will be quite a bit higher than 100C - though that does not mean that opening up a hot pressurised coolant system is a good idea.
For what it is worth many people comment that their cooling system level changes dramatically between cold and a hot engine, none of my cars have ever behaved like that, though maybe I don't also only end up to continuous nose to tail traffic.