Why are part year residents taxed more?

Even though this is the tax fourm and no tax advice is given I have a question I would like to ask.

I lived in two states in 2007 so I need to file a non-resident/part year res. for each state. What I did was a fed form for just one state and the single form for the state. I compared it to the non-res/part-res and there is a difference of 10K in taxable income? It is also the difference almost $600 in refunds. So why do part year residents get taxed more? Is there a way to actually report the states seperatly that show what should be taxed?