Yes, it is quite simple actually.
It is called
FeatureFreeze, and it means at a certain point late in development they stop updating
ALL applications, and concentrate on
stability and
bugfix-updates only.
This means, for example, critical applications like browsers get updated regularly, but not much else.
A large part of the software on a Ubuntu computer is out of date. With calibre it is noticeable, as calibre is updated weekly.
If you were using the Ubuntu 15.04 betas, you'd probably have an up-to-date version of calibre.
Ubuntu is not the only distro that does this, it is actually quite common. There are distros, like ArchLinux which I use, that keep on the bleeding edge using the
Rolling Release model. There is no "ArchLinux XX", like there is "Ubuntu 14.04", rather, you update to the latest version by applying all package updates.