[CentOS] 64 bit and 32 bit are mixed up

Hello

I have installed Centos 5.1 on 64 bit servers, and I see some of the 
packages have been installed two times,
one for i386 and one for 64 bits. I don't know why i386 packages can be 
installed on 64 bit machine and why they
should come together, even some times when I am searching yum repository 
, I see i386 and x64
packages. shouldn't the search result depend on server architecture ?