When the VA's audition for the characters, they haven't heard any of the japanese dub. The producers make a decision on what voice they feel best suites the character. After the audition process, a few weeks later, the VA is told he's got the part. The first 4 scripts, as well as the original japanese dub is made available at the studio. If any of the VA's choose to pick it up is another thing. A lot of VAs don't. I did..
When the VA shows up on the first day, they are played their original audition that actually got them the part. The audition that was recorded before they even knew what the series was about, let alone had listened to a dub. So ya... Thats why. :P