It does seem odd that there's friable soil under the clay. What it sounds like is that they had put down some potting soil, then covered it with 'real' soil, and your real soil has lots of clay. Potting soil has no business being in an outdoor garden. That's not what it's formulated for.We just moved into a new house in northeastern Tennessee and it came with five raised beds. So, we started weeding them as they were left in a horrible condition by the previous owners. The soil here is filled with a LOT of clay so I have to do something about that. In two of the raised beds, as I dug down to get out weeds, I found some soil underneath the clay that seemed to have a lot of Perlite in it and it was rich and dark so I'm not sure what the deal was there. Seems like it should be the other way around. I wonder if I could just add some new planting soil over the top of the old stuff and add new Perlite and then perhaps add some bone meal for phosphorus and then maybe top it off with a light sprinkling of Epsom Salt for magnesium. What do you think? We are very new to this and so we aren't real sure about how to amend this clay other than to maybe cover it with good soil.
Putting more soil on top isn't the way to go, even if it's 'planting' soil. There are some bagged commercial 'garden soils' that are meant to be dug in, but those really aren't that great, and even with those, you should not just put it on top.
For an immediate effect, you can add a commercial, bagged 'soil amendment', but you'll have to dig it in and hopefully also incorporate all 3 layers. For a long term effect, put compost, mulch, leaves, etc. on top of the soil and the earthworms will dig it in for you (long term).
If you have lots of weeds and other growth that you aren't able to get out, cover it with a couple of layers of cardboard, wet thoroughly, and then cover with about 2-4" of organic material (compost, mulch, leaves).
Raised beds have a tendency to shrink down every year (don't ask me why the soil in the ground doesn't also shrink, I have no clue), so you'll have to add to the top of the beds every year. But don't do as the previous owners did. Primarily add organic material. You can use a little soil, but be sure to mix everything into the top couple of inches.
I wouldn't just automatically add bone meal and Epsom Salt. At a minimum, get one of those NPK soil test kits at the local nursery or building supply store, so you can see what the balance currently is, before you start adding stuff.