How exactly is it behind? I didn't notice it. If you mean it doesn't offer half cooked buggy software for everyday use - I don't call it behind. I'm using Debian testing for regular desktop needs and it's a decent balance of stability and up to date features.
At times I do feel like packaging in Debian lags because of maintainers lack. For example KDE could get packaged faster (there is no Plasma 5 in Debian yet). But usually it catches up if it falls behind.
I guess that I mean, it doesn't offer half cooked buggy software. :-) Well, at least it works about as good as Windows stuff. This is from many years past, so that's the experience that I've had. But as other commenters stated, and I didn't know, the unstable branch is much more current than I would've expected.
At times I do feel like packaging in Debian lags because of maintainers lack. For example KDE could get packaged faster (there is no Plasma 5 in Debian yet). But usually it catches up if it falls behind.