question about revision numbers in svn

[alex wallis]

Hi list.
I was just wondering if someone could answer a question for me about revision 
numbers in svn.
I notice quite often, that revision numbers often get skipped, meaning that 
some times there can be several numbers missed out.
I was just wondering why this happens if svn is supposed to track all changes.
An example of what I mean is there is a jump from r.19212 to r. 19210 with no 
r. 19211 showing up in the recent changes list. I just wondered why this 
happens?