The problem is with the way the version of a build (especially dev version) is displayed.
In Lubuntu Quantal, the LXPanel version that works with only
libmenu-cache1 is displayed as
But in Synaptic Manager it's 0.5.10:
~$ apt-cache policy lxpanel
*** 0.5.10+git20120823-0ubuntu1 0
500 http://fr.archive.ubuntu.com/ubuntu/ quantal/universe i386 Packages
I do not know how Launchpad displayed the name of this version, but it could be a dev or otherwise improved version of 0.5.10, although I do not remember manually updating LXPanel in Quantal.
Also, after installing (in Lubuntu Precise) LXPanel for Precise from here, which looks like being a daily build of 0.5.11 (Launchpad named it lxpanel-dev-0.5.11~bzr1306+201301150740~precise1) (which was possible only after installing
libmenu-cache2 for Raring from here) I see that LXPanel "About" says version 0.5.12, while in Synaptic it was displayed 0.5.11.
So, the dev version installed in Precise is not the same version as the one from the other computer running Quantal, but a newer build that asks for libmenu-cache2, while that quantal version does not.
It seems that LXPanel versions above 0.5.10 need
(But to see the real version of a build one must look in Synaptic or run
apt-cache policy lxpanel, because dev builds might show a newer version in "About" section.)