The difference is simply this:
A car that takes 11 seconds to travel down a 1/4 mile strip, takes 11 seconds.
A car that makes 300rwhp on a dyno, doesn't make 300rwhp, because almost none of the machines out there calculate hp correctly. At the very least they don't calculate hp the same! A dynojet horsepower is different than a dynopack horsepower, is different than a dynocom horsepower.
Now imagine if every 1/4 mile strip used a different "second" as a measure of time, and then trying to compare 1/4 mile times to each other... that's where the dyno world is right now... and consequently why I speak against this kind of comparison.