When it comes down to it the difference is because of where/how analytics are measured between the two sources.
SS Stats is gathered from a redirection step we take from forwarding your original media file URL to the ultimate destination of your media file (whether you host with Castos or not, this is how we are able to give stats for all SS Stats users). Of course as you can imagine with WP this will behave a bit differently for everyone.
Castos Analytics are measured directly from our CDN provider, which measures the actual requests of media files on our platform. We do some similar filtering out of crawlers, bots, etc. as well as duplicate/partial file requests.
In the end I can’t say that one is more accurate than the other, but I can tell you that we’re doing more work on Castos Analytics to improve the amount of data we display to you there, so that’s where I’m basing all of my decisions personally for my shows.
Hope that helps and if you have any other questions just let us know.