Taxpayers assume that 4a should be the total amount of IRA/pension/annuity distributions and 4b should be the taxable amount, but that is clearly not what the IRS instructions call for.

You can see for yourself on pages 28 and 29 at https://www.irs.gov/pub/irs-pdf/i1040gi.pdf for lines 4a and 4b.

"Except as provided next,
leave line 4a blank and enter the total
distribution (from Form 1099-R, box 1)
on line 4b."

There are multiple exceptions listed ("except as provided next") in the text that follows, in which you are supposed to add the total distribution into 4a, not leave it blank as stated above. This means that sometimes the total distribution of an IRA/pension/annuity is added to 4a and sometimes it's not.

This was tolerable while IRAs were reported on one line and pensions/annuties on another (yes, this is the way it has worked for years), but in 2018 when the IRS redesigned the 1040 form, they lumped IRAs and pensions/annuities together on one line.

Now we see much more often that some of the total distributions are added to line 4a and some are not, with the result that 4a is sometimes smaller than 4b.

This surprises a lot of taxpayers, but it is exactly what the IRS instructions call for.

The TurboTax worksheet shows the actual numbers, but the form 1040 (which is the filed form) shows the numbers as the IRS wants them.