I noticed a few things about this, one of these might be causing inaccurate records:
1. If you play the same game more than once during one day, it will likely be recorded as just 1 time played.
2. Any play time accumulated when the Wii is not set to the correct date and time will not be added to the total.
3. Only games listed in the Nintendo Channel database will be logged.
4. The WiiWare demos are not recorded by the channel.
5. The Mario Kart Channel time is combined with the game itself.
Sometimes it really does seem random though.