[Yum] A brief yum-rpm question

[Daniel Romeo]

Hi!
When I try to install (by way Yum check or update or install) SeaMonkey 
1.0.3-0.el3.1 over a RHEL 3.0 AS with mozilla-1.7.13-1.1, the operation 
show a couple of inconsistencies and cancel installation. Then after, 
installing Sea in direct rpm way, I see that the correct option is -Uvh 
(to upgrade, and no -ivh to install). The question is, Why for that yum 
did not find the difference?

Greetings, Daniel.