I'd like to know too. 7.0.1 has schema version 2.0 as well so it doesn't seem to change with major revs either.
My guess is that SchemaVersion refers to the xml format of the BLPackage file in the depot and not a database schema.
The contents of a BLPackage xml file have SchemaVersion and the oldest package I could quickly find has the following entries:
<BLPACKAGE CreatedBy="joe" CreatedDate="03/24/2005 22:40:03" SchemaVersion="2.0">
SchemaVersion 2.0 seems to have been in use for at least 16 months, probably more.