Not necessarily. That's one thing I want to find out.
The Nintendo Channel must use more values than simply adding up the play history messages on the message board, as games played when the Wii was not set to the correct date and time won't be logged. For example, my playtime for Excite Truck adds up to around 15 hours according to my messages, but the Nintendo Channel shows it at only 1 hour, 20 minutes. I've no idea if this was intentional or not.
So, if it can skip over time, I wonder if it's possible, through maybe a bug, that it could sometimes count a day's log twice.