Although the major number is lower (19 vs. 20) this doesnt neccessarily mean that its's older. Debian version numbers and upstream version don't match in many cases.
The version in sid is: 0.19+20121004-1 which indicates that it is based on source version 0.19 plus cherrypicked patches until October the forth last year. Ok, five month ago. But with Debian in freeze we all have to wait for normal workflow until after Debian/Wheezy is released.
And see ubuntu version: 0.19+20130117
Still v19, but nearly up to date. (however, Ubuntu has no freeze.)
For detailed info about what blocks this package from being updated see:
http://packages.qa.debian.org/b/btrfs-tools.html(which boils down to what I wrote)