The currys served on the street are never, ever as good as the currys in a restaurant. They both seem to use the same ingredients, so why is a restaurant so much better? One thing I wondered was that there was more water in the currys compared to broth or milk or cream in a restaurant....thoughts?

Street curries are usually thicken by traffic air.

Some street curries are as good as, if not better, than many restaurant curries. It all depends where you go. Generally, in my experience, street stalls serving southern Thai dishes are better than those serving central Thai food.

That said, some street vendors use very poor quality ingredients to keep the costs as low as possible. There have been horror stories of chicken meat that has turned green being treated with chemicals to make it appears normal again and the like. There is also a question of technique. A good, coconut-based curry, will have the curry paste fried in the cracked coconut cream. As a short cut some places will instead fry the paste in oil because it's quicker and easier at the expense of the taste of the finished product. Then there are the pastes themselves. You can buy a commercial curry paste, which will have been milled, rather than pounded, or you can pound your own paste which takes ages - particularly if you're doing it on a commercial scale. Your paste may just contain a handful of ingredients because it's easier, or it may contain a couple of dozen ingredients because you want the dish to taste correct.

I think you also tend to get more meat in restaurant curries as opposed to street curries. That's reflected by the price, as well.

I think it all comes down to whether the cooks pound their own fresh curry paste or buy commerically prepared, preserved pastes. Most - but not all - street khao kaeng places use the latter.

Near the office where I work there are two khao kaeng stalls where a plate of rice and curry costs 25 baht; both use ready-prepared pastes and the taste is pretty uninspired. Across the street is a small sit-down restaurant where the cook - a Muslim Thai woman (although it's not a southern Thai restaurant - she's from Minburi) pounds her own curry past fresh every morning before preparing the curries. A plate of rice and curry costs 40 baht there but there's a world of difference in the flavor.