This is more than likely because your cable provider is cheating and using a dual connection to provide you your full speed.  My cable provider does the same and I'm using an SMC Docsis 3.0 modem.  The dual connection definitely is not a very good solution either.  I ended up downgrading to the 2.5MB service with the 1MB up.  In doing that, I see closer to 3MB down and 1.2MB up because of similar issues that I relate to the way the cable companies deliver the speed.