income tax rates for corporate income 对不同的收入有不同的marginal tax

income tax rates for corporate income 对不同的收入有不同的marginal tax rate. 为什么会有不同?
不同taxable income中,marginal tax rate从15%,25%,34%,39%,34%,35%,38%,最后到35%.
其中,为什么一开始从15%一直升到39%之后又下降了呢?为什么后来又略有上升?
（is to make sure that firms that have very high taxable income don't benefit from the lower rates on the initial dollars that they earn.）《——书上的原话,可以具体解释一下或者举例说明一下吗?
谢谢
核心问题：为什么marginal tax rate从一开始的15%升到39%后，又有小浮动变化？