According to http://xubuntu.org/news/xubuntu-14-04-beta-1/, light-locker is supposed to replace xscreensaver for XUbuntu 14.04, but /usr/bin/xflock4 uses xscreensaver in preference to light-locker if both are present. Since xscreensaver is also installed by the default install (or possibly by some package I installed - but it's not something I explicitly requested) this leads to the situation where the display is locked by light-locker if due to a inactivity, but by xscreensaver if one hits a lock button or key shortcut.
Presumably the default order should be changed in xflock4, but also XUbuntu shouldn't install xscreensaver at all.
ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: xfce4-session 4.10.1-3ubuntu3
ProcVersionSignature: Ubuntu 3.13.0-16.36-generic 3.13.5
Uname: Linux 3.13.0-16-generic x86_64
ApportVersion: 2.13.3-0ubuntu1
Architecture: amd64
CurrentDesktop: XFCE
Date: Tue Mar 11 21:22:19 2014
InstallationDate: Installed on 2014-03-10 (1 days ago)
InstallationMedia: Xubuntu 14.04 LTS "Trusty Tahr" - Alpha amd64 (20140225)
SourcePackage: xfce4-session
UpgradeStatus: No upgrade log present (probably fresh install)
Status changed to 'Confirmed' because the bug affects multiple users.