Your computer's firewall/anti-virus/security software interferes with the connection.If anything, a speed test will tell you the theoretical maximum speed that you could stream at under perfectly ideal conditions, but conditions are never perfect. Your "stable" bitrate is more likely about 70-75% of your "estimated" speed test upload (and that's only if you're not being throttled).
Just because a speed test says you have 5 Mb/s upload doesn't mean you can upload to anything at a stable 5Mb/s since connections across the internet rarely maintain such a consistent speed. Speed tests provide very rough estimate - they mean very little with regards to streaming. If you drop too many frames, you may be disconnected from the streaming server. Because of this, the program was forced to drop some of the video frames in order to compensate. "Dropped frames" means that your connection to remote server isn't stable or you can't keep up with your set bitrate. Check your anti-virus/internet security software.