It is true that there is wide spectrum of opinions when it comes to interpreting the Bible. Different opinions are not unique to biblical studies – scholars disagree about how to interpret Plato or Hume – but the disagreement is often far more pronounced amongst biblical scholars. One reason for this is that people have a lot invested in the results of biblical scholarship. If the Bible is the Word of God, and contains ethical and spiritual instruction, then understanding the message is very important. So no wonder people sometimes disagree. If there are certain things I need to do or believe to be saved then that is a big deal and I will want to make sure I have understood those things correctly.
There is always a danger that we try and interpret the Bible to suit our own beliefs rather than letting it speak for itself. For example, if I believe that Christians need to keep the Sabbath I will put a lot of emphasis on those passages which talk about the importance of the Sabbath, and maybe play down those passages which say that the Law no longer applies.
The other danger is that we bring our own presuppositions to interpreting the Bible. For example, if I believe that miracles are impossible (perhaps because I am a materialist or because I think that God does not intervene directly in the world) then I am unlikely to interpret the miracle-stories in the Bible literally. I might choose to interpret them allegorically or regard them as simply false. But that is not what the Bible is saying, that is simply my interpretation of the Bible. Scholars often disagree about the interpretation of the Bible because their presuppositions disagree.
A good bible scholar should reveal his presuppositions and be consistent. He or she cannot simply pick and choose, saying that bit is literal and that bit is allegorical. First he or she should decide how to tell the difference between what is allegorical and literal, and then apply that principle to interpreting the bible.
In summary, bible scholars disagree because they have a vested interest in certain interpretations being true and because they approach the bible with different presuppositions. This is the position that we all find ourselves in. The best we can do is be honest with ourselves and ask why do I believe such-and-such, and is that actually what the Bible is saying?