Bet volume Question

I don't quite get why the system is set up in such a way that if I place a bet at 1.01 to win £10 it will show up as a bet volume of £1000 and if I lay the inverse at 101 to win £10, it will show up as a bet volume of £10 when the bets are identical.

In each case the money that has been matched amounts to £1010.
Yet if you look at the two charts after these transactions one will have increased in matched volume by £2000, and the other by £20.

Because the amounts matched on selections that are each others inverse will never be exactly equal, the matched amount shown becomes skewed and no longer reflects the true value of bets matched, which would be easy to display.