I’m not sure why this is so confusing. It appears that Democratic style policies (social programs, progressive taxes, some market regulation) result in a more productive economy. If we accept that as true (which seems to be the real sticking point) it really makes sense that the leadership imparted by the President would have an impact on the legislations passed.
We’ve seen what kind of sway our current President has when threatens to veto bills, or is vocal in his support (or lack thereof) for a bill. He may be an extreme case, but it seems reasonable that any President would exert pressure or influence on the creation of a budget, or other bills impacting how the government will spend its money. Republican Presidents will support policies like regressive taxes, deregulation and decreased social spending, while Democratic Presidents will support policies like progressive taxes, social programs, and regulation.
Obviously, trends over the last century indicate that the Democratic policies create a more productive economy. If that’s true, it makes sense that the President will have that kind of sway over the legislation and spending that becomes law.
I think this just reinforces the idea that Democrats are better stewards of the economy. We really shouldn’t be shocked