The changelog on this package will explain it. The current latest entry:
lxpanel (0.5.10+git20120823-0ubuntu1) quantal; urgency=low
* New upstream snapshot.
- Lxpanel doesn't use monochrome Keyboard LED indicators (LP: #1016014)
Means that the Ubuntu developers (package maintainers) have taken a new upstream snapshot of
lxpanel to fix a bug. While this may not actually be 0.5.11 yet, the source of that snapshot will already contain new commits after the 0.5.10 was released and the state of the source code repository might already be in a prepare-for-0.5.11 state at that time.
A common reason for naming the package version slightly different is the current set of dependencies relying on a 0.5.10 version available, but if the ABI compatibility hasn't changed, the 0.5.11 version just works and naming it 0.5.10 in the package management saves a lot of trouble in shipping out more updates with just a line of dependency changed.
However, if package maintainers decide on this, they usually version tag it like this:
4:4.7.0really4.6.0-0ubuntu1. In this case the 0.5.11 version might not have been out yet, so they just use the
git20120823 suffix to mention the upstream version not yet released.