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?

题目

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？

答案

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.是为了确保那些纳税收入额非常高的公司对他们最初赚的钱不享受低税率...