this seems to be the case since at least beta 5, and apparently still present. Suddenly the numbering of the new, untitled documents begins to jump, and so I'm at untitled 86, then it's 295, and it keeps on skiping numbers. (the numbers here are examples only)

I don't really know what triggers it, but my usage pattern is really simple: I mostly just load a lot of sketch projects from the terminal, and every now and then create a new, non-project file.