Because Hybrid theory was the name of the band before they were called linkin park and an EP is an album that has (I think) 4 - 8 tracks. It was released first because it was the demo they sent off to record companies to get a record deal and half the songs on Hybrid theory werent even made yet. That was a pretty crap explanation. sorry.
An EP is generally a shorter CD. It has nothing to do with the "extension" of a CD. There is no such thing.
Oh so your saying that whne they made this album they were still called hybrid theory? Cool,better than xero
Okay, this is the short of it, EP is like half an album and LP is a full album. The EP is normally or demo version or sample version of the LP, to give people a taste of it. Although Linkin Park changed it completely, Rappers use the EP to give the people a sample of the next cd. It's also away to make more money if you mass produced both.