Synths are handed to server instances on a first-come first-served basis, but because we have many instances in parallel, it's possible for s shorter one to finish before a longer one that was uploaded early.
On the issue of an eternal "Waiting to be Processed" message. We have a bad bug in our code right now that I'm pushing to fix ASAP. The state we show when an upload is in progress should be "uploading", but currently it's not. It's "waiting to be processed". Compounding this problem, due to http issues, we can't really know if the upload has been abandoned (say you navigated away from the page, killed your browser, or rebooted your machine).
We need to show the state correctly, and allow you to tell US that the upload has been abandoned.
And before you ask, we're not planning on writing anything in the near term to allow people to "continue" an upload in a different session. Useful, but not high on the priority list right now.