The biggest impact will be from rain leaching nutrients down. Organic matter will also very slowly make its way down as it breaks down. The mats of grass you turned over will take a long time to break down. The next time you build a bed remove the grass and lay down cardboard. It will surprise any weeds and draw in worms. For some reason they seem to love eating cardboard.

Regarding the grass breaking down. The time it takes depends on the type of grass. Turning the turf over works fine with many types of grass. Rotted turfs are a great soil conditioner.

I wouldn't even remove the grass first, just mow it low and lay down the cardboard or newspaper on top. Anyway, yes, the raised bed is improving the soil below. Worms will go after your highly organic raised bed soil and in the process they'll leave worm tunnels in the soil below which will speed up the transfer of organic matter downward. It's a slow process but it does happen. Try digging a hole and see what's down there, before you plant for next season.

It's OK to have some mineral soil in your raised bed in addition to compost. Some people grow in pretty much all organic container mix or compost. I'm not going to say it doesn't work but it's not a container so it's OK to add some soil. If nothing else it will keep it from shrinking so much every year.

I'm about to start building a new raised bed and I was debating between 11" and 15" but I think I'll stick to the 11".

Thanks for the input.

Another thing you can do, especially if you have heavy clay soil underneath, is to break up the surface with a shovel before adding soil on top to make the raised bed. You don't have to mix it in, but chopping the top few inches into a rough surface will help provide a transition zone from raised bed soil to the native soil below.