By itself, this discrepancy doesn't prove anything; however, when you consider it along with all the other evidence, it's just one more reason to doubt the US Government's story.
You guys like to take problems like this, say, "it doesn't definitively prove anything on its own, because there are other possible explanations for it -- debunked", and then completely disregard it from that point forward. In other words, you fail to examine the entire body of evidence.
Take, for instance, a murder trial. The prosecution has multiple eye-witnesses, as well as physical evidence to support their case. You can look at the eye-witnesses and say, "there's a possibility that they're lying". You can look at the physical evidence and say, "there's a possibility that the police are framing the defendant". When you consider everything together, there just isn't much of a chance that both of those scenarios are true.
So, yes, you guys are correct - when you isolate this latest problem, it doesn't definitively prove anything. Add it to the list of other problems, and it's just another reason why a new investigation is absolutely necessary. If you don't think so, I suggest you ask yourself why you're so opposed to a new investigation if you're so sure about what happened.