It's probably some form of hard load-balancing that they are doing based upon time period. As it stands, FAP is technically "as low as, or lower than 1Mbps", where as we also have FAP where it's 1-3Mbps depending on congestion.
I would say the 5Mbps you saw, was what's considered overhead since there wasn't much traffic for you apparently.
I was lucky to see 2Mbps last night, WITH data.