I think you are referring to the accumulated odds? The odds in your favour obviously decrease as soon as you start accumulating them with other odds of good value. So I for one usually never pick more than 4 teams where I am getting very good odds. I never bet against Oxford as that to me is a bridge to far (could never root against the yellows).
If you just look at the face value most punter will look at the odds and think, thats not too bad in football terms. Currently Oxfords odds of winning this game are 13/20 a big difference from the true odds, so basically you should never bet at those odds particularly when the true odds are a lot different, whereas Burton are 7/2 which is a great price against the true odds of about 4/5. The Portsmouth odds are a lot closer to the betting shop, but still worse than the true odds