App shows as open in the dock, but isn't shown in Cmd-Tab

There's a bunch of niggling things about Yosemite & El Capitan that just don't seem to work very well.

One is that the Dock shows some apps as being running, like Preview & TextEdit as I type this.
But if I Cmd-Tab to cycle through open apps, they're not there.

Not the end of the world or anything, but a bit tatty..