mtgox fees appear to be inaccurate.   I thought they charged 0.65%

Here is an example:

bought 2 BTC -  fee charged: 0.01300000 ฿TC
bought 1 BTC -  fee charged: 0.00600000 ฿TC   

They undercharged me on the 1BTC trade. (should have been 0.00650000  ฿TC)

I also saw some undercharging on USD values when selling.

So far these minor miscalulations work in the users favour - so I guess nobody is really complaining - but have mtgox said anything about this?
If they're not accurate in charging these fees.. what else are they not accurate in calculating?