As I wrote on XEmacs-Beta:

For example, if the APIs that use a customization change, but its 
default value doesn't, then :version doesn't change but its meaning 
does.  I'm not even sure if they update :version when additional 
non-default values are added to a customizable variable.  Also, if a 
variable is deprecated or deleted, that won't change :version AFAICT. 
Finally, the most interesting changes are in functions, but those 
don't have a :version at all! 
 
Might be interesting to implement that, if it can be done 
automatically.  I wonder if hg has a git diff equivalent (ie, that 
reports the function containing changes) that could be warped into use 
here, eg as part of the DOC file.