AFAIK libdvdcss2 is just the name of the latest libdvdcss package. The latest version of libdvdcsss is 1.2.10. Presumably there used to be an older "libdvdcss" package which was superseded by "libdvdcss2" - there might be a known reason for this.
For example, there are two packages of apache:
http://packages.debian.org/search?keywords=apache
apache being the old Apache 1.3.34,
apache2 being the current 2.2.x versions.
The libdvdcss2 package name has been around for a while anyway:
http://downloads.videolan.org/pub/video ... 1.1.0/deb/
Arite.