Part of that soil is natural compaction and part of it has moved deeper down into your native soil thanks to roots, worms, bugs, etc...

You've now got a nicer soil profile below your boxes and it should only get nicer.

You can top with pretty much any "soil" you want as long as you add some organic matter with it. The compaction issue should be a lot less as your beds age.

Yes, use the same thing you put in there the first time. It's best to keep the texture uniform!

if you have access to leaves and or garden grass you might add some. look into home made compost. look into lasagna gardening methods. you can also do a search here on garden web.

Here is a link that might be useful: lasagna gardening info

Now is not the right time to add leaves and grass clippings.
You have to do that at the end of season, so that the micro organisms
and worms get to work and brake them down.
At this planting time you can add compost and composted manures of all kind.

I beg to differ with adding the same stuff you used last year. More than likely the organic matter (compost) has broken down. Dig up some of the planter and really look at the soil. If you can't see anything but soil than add compost and mix well. Since your beds are so deep it's going to be a deep dig. We have the same problem at our community garden and the first year we added a soil blend back in....big mistake. The next year the beds hadn't dropped and I realized that all the organic matter was gone. Now we add compost every time the beds drop and the production has been much more consistent. To keep from having to kill ourselves with huge loads each time, we add it in the spring and fall and mulch the beds that aren't going to be used for winter gardening with compost.

I also add some sort of fertilizer to get a wee bit ahead of the plant's needs. That way I don't deplete the soil at all with the coming crop and the microbes can be happy too. Not a huge amount of anything and I use a different type every year just to keep everyone on their toes.