My best guess in my knowledge, is the malware definitions are being compressed (presuming it will save space and make the program run more efficiently).

For example, in 2007 there might be Malware A. Spybot incorporates that definition into its definition database. Now in 2011 there are several variants (versions) of Malware A. So instead of searching through:
-Malware A-v.1.2
-Malware A-v3.1
-Malware A v.0202
etc...

All the definitions for those variants are compressed into one general category: Malware A.

Hope that helps?