No way to know, the speed throttling is only done if you are CLOGGING the port I don't think 6.3 Mb/S upstream is doing that but that depends on your CMTS, call in and ask, more likley your server is coming under load and not able to receive as fast.
Most of that is what they call power boost, where you get a chunk of unused bandwidth at the start of an upload or download for like 10 MB, after that its back to the sold speed. So if you bought say 20/2 and started the upload you pegged 6.3 but then after 10 MB of send, it goes back to the 2Mb/s you were sold, and its pretty spot on for that.
If you want to know try learning how to use shaper probe and run it.