A note on dimensions: U-slots are generally formed such they are not square. i.e., the cross-section dimension at the top will be smaller (by only a few thousandths) than the same dimension at near the bottom. This is to facilitate clamping/locking mechanisms so that lateral force tends to pull tracks down into the channel; that is, locking bars will wedge themselves in a bit. This is similar to how clamps and dogs can have surfaces offset a few degrees to keep work from squeezing out, or losing square as force is applied. U-slots also tend to have generous corner radiuses in the bottom to allow for free movement in the slot, and to allow room for locking mechanisms.
19mm is roughly 3/4 inch, and these slots are meant to be a bit sloppy, so perhaps the Euro sizes correspond to those, as well. 13mm is a little sloppy for 5/8ths, but it might be close enough. I mean, it depends if they are talking about the slot or the track, and if the track is meant to slide or hold during use.
Because those uA note on U-track purpose: U-tracks have two purposes: you either want something to slide with a slight interference fit to act as a guide (e.g., a cross-cut sled, or mitre bar) or you want it to clamp and not move (e.g., finger-boards or other safety equipment). Sliding bars want a loose interference fit so things stay square. Locking bars can be a little looser because they have a locking mechanism (with a j-slot, or a cammed t-nut, or some other mechanism) that is intended to keep the bar from moving.