Line 39a and the Standard Deduction Worksheet

It's correct.

And one of the first things I learned about tax law is never ask Why.

This goes back to the Tax Reform Act of 1986. All I can tell you is that the House and Senate versions of the amended bills both had an additional std deduction for blind and age of $600 for individuals. When the bill came out of conference committee, the conference report showed the $600 amount was still there except for singles and HoH, where it was $750. You would have to go back and try to find some explanation as to why the conference committee changed both the House and Senate versions of the bill. I would only be speculating.

That is not a mistake. The amount varies according to marital status, not filing status per se.

It just seems odd to me to give the same, enhanced deduction for someone who is single as for a head of household. I guess that means marrieds get a slightly lower deduction per person.

HoH can be filed for those who are married and, in certain situations, by those still married (the former "abandoned spouse" terminology.)

So I disagreee that the deduction goes with marriage status and not filing status.

IRC 63(f) defines the additional standard deduction in terms of marital status:

"(f) Aged or blind additional amounts "(3) Higher amount for certain unmarried individuals "In the case of an individual who is not married and is not a surviving spouse, paragraphs (1) and (2) shall be applied by substituting "$750" for "$600". "

(Amounts from 1987 before inflation adjustment).