Why do progress bars animate backwards?

Gmail’s new loading bar drew my attention to this today:

Gmail loading bar

The colored pattern inside the bar animates from right to left.

The Mac OS progress bars do the same:

Mac OS progress bar

Is there a reason for this? If I was asked to animate a progress bar I’d automatically do it the other way, so I’m assuming it must have been a conscious decision.

Update

I recently read an article that included other ways of making a progress bar appear faster to users:

  1. Increase the Number of Pulsations – “The progress bar with increasing pulsation was more likely to be perceived as having a shorter duration”

  2. Accelerate the Progress and Avoid Pauses at the End – Progress bars “with the fastest progress occurring near the end of the process” were perceived faster than progress bars “with pauses near the process conclusion”