When two bets are matched, the odds are always determined by the bet in the pair that was first placed.
ok now i'm confused, what do the numbers on there actually mean? i assumed those are the odds, so if it would be for away back/lay 10/10, that would mean 50:50 odds?
Those are the odds that you've selected; they aren't the only odds you are willing to take, they are, rather, the worst odds you will take.
If someone made a bet that with a wager of 8 and and a counterwager of 10, his bet wouldn't match yours, since those are worse odds than you specified in your order. If, however, someone made a wager of 12 and a counterwager of 10, he would match your order. And if your bet was made first, the actual odds of the bet would be 1:1, whereas if the counterparty made his bet first, the odds would be 5:6 (for you, and 6:5) for him.
The logic for matching bets is exactly the same as the logic for matching orders.