OCZ packs these modules together, and they claim to have tested each module. They are placed in dual kits of matched pairs for optimal performance and compatibility, especially in a dual-channel configuration. OCZ is not the only one to do this. As a matter of fact, a quick scan of many other manufacturers proved that almost all of the major brands are released this way as well. It certainly made sense to do this.
The “EL” stands for Enhanced Latency, and is responsible for allowing these modules to reach speeds of 434DDR and still maintain a Cas 2 rating.