The answer about what your refund should be is either wrong or value judgement about what your refund *should* be, not a fact about withholding tables. For example if a single person has a $70k W-2 job and no other income, they would have a refund of $987 if they file their W-4 with one exemption. If they claim two exemptions on the W-4, the tax due would be exactly zero. The tables withhold more than necessary so that most W-2 earners won't owe money because the government knows most people are terrible with money. Withholding tables get more complicated on joint returns though.
That's incorrect.
(I'm going to assume that what you assert about the tax refund amounts based on allowances (they're allowances, not exemptions, on the W4) is correct. It seems right to me but I don't want to go through the tax withholding tables right now).
If a single person earning $70k fills out his/her W4 as the instructions say, then he/she will claim two allowances: one for himself/herself because nobody claims him/her as a dependent, and one for being single and having only one job.
As you say, then this would result in a tax refund of zero. The tables aren't withholding more than necessary, they're exactly right assuming no other sources of income.
I fail to see how this question is a value judgement about what your refund *should* be.