deb-version - Debian package version number format
Version numbers as used for Debian binary and source packages
consist of three components. These are:
epoch This is a single (generally small) unsigned integer.
It may be omitted, in which case zero is assumed. If
it is omitted then the upstream-version may not contain
It is provided to allow mistakes in the version numbers
of older versions of a package, and also a package's
previous version numbering schemes, to be left behind.
So, that extra number (in your case
1) refers to the epoch component which may be omited in which case 0 is assumed. And so, if you see a version string which looks like
1.6.2-0ubuntu3 you can think that in fact it looks like
0:1.6.2-0ubuntu3. How is this helpful and how does this comes: It is provided to allow mistakes in the version numbers of older versions of a package, and also a package's previous version numbering schemes, to be left behind ? To understand better, take a closer look at the following explanatory paragraphs from Debian Policy Manual - Control files and their fields:
When comparing two version numbers, first the epoch of each are
compared, then the upstream_version if epoch is equal, and then
debian_revision if upstream_version is also equal. epoch is compared numerically.
Note that the purpose of epochs is to allow us to leave behind
mistakes in version numbering, and to cope with situations where the
version numbering scheme changes. It is not intended to cope with
version numbers containing strings of letters which the package
management system cannot interpret (such as
with silly orderings.