My balance is the same as it was before I put those bets on.
I don't use betfair.
I'll draw it out nice and simple for you champ. Assuming you bet the same amount on each, it works out as follows:
1 unit: Mercury -4.5 @ $1.91
1 unit: Mystics +6.5 @ $1.91
Total spent: 2 units
Total returned: 1.91 units
Net return: 0.09 loss (this is your commission, dingbat)
The only way possible that your account balance could be the same as it was before you (hypothetically) put those bets on, would be if your bet on the Mystics was 1/0.91 times larger than the bet you put on the Mercury.
On betfair, your commission would be lower.
Did you even finish high school Bolar?