rpm version numbering

Hello list,

I still can't figure out how the version numbering works.
I have for 6.2 and for 7.3 channels with all RedHat RPMS, all RedHat updates, 
powertools and updates and last but not least our own rpms.

One of the set of own rpms is postfix.

In the 6.2 set there are these rpm's:
postfix-19991231_pl02-4.i386.rpm		(6.2 powertools)
postfix-1.1.10-1.i386.rpm			(own set)
current says that the 19991231 version is the latest, which I can understand, 
because this is the largest number.

In the 7.3 set there are these rpm's:
postfix-20010202-4.i386.rpm			(7.1 powertools)
postfix-20011125-1SASL.i386.rpm		(7.1 powertools updates)
postfix-1.1.7-2.i386.rpm			(7.3 base)
postfix-1.1.10-1.i386.rpm			(own set)
current says here that the 1.1.7 is the latest, how can this be?

can anyone explain how this works?

Jacco