Statistics help required re the National Lottery

The Telegraph have a helpful table of the winning lottery numbers for each of the last 15 year's lotteries.

Now because I am a bit geeky, I put the figures into Excel to do some analysis but right away I spotted something odd. If you add up all the times that each ball has been drawn in the main draw you get 8,742 which when divided by 6 (the number of drawn balls in each main draw) gives you 1,457 draws. However if you add the number of times each ball has been drawn as the bonus ball (one per draw) you get not 1,457 but 1,451 - WHY?

I refuse to do any analysis until I have solved this basic query!