Betfair over/under round question

[DublinMeUp]

Hi Guys, Just a quick question, mainly due to my knowledge of laying being pretty small. So, assume we have a race on betfair at 101% on the back side and 99% on the lay (without commission), this means that if we convert every price to a percentage on say the back side we will be left with each horses perceived chance of winning (1% extra noted). If we do the same for the lay side we will no doubt get very similar percentages, similar being the key word here ie not the same. I know in this particular example the differences will be miniscule but still; which side is theoretically more accurate? Could you work out the 2 and average the percentage? Just something that's been bugging me today, thought someone may have a quick answer. Cheers