This can happen because of the incomplete IRS instructions for line 4 on the 1040..
In the past, IRA distributions and pension/annuity distributions were reported on different lines. The IRS instructions said at that time (more or less) for both IRAs and pensions/annuities that if 100% of your distribution was taxable (the equivalent of 4b), then you didn't have to enter the total distribution (the equivalent of 4a) but just leave it blank.
Now in 2018, the IRS has combined IRAs and pensions/annuities into one line (4) - but they didn't rewrite the instructions. That means that it is unknown how the IRS wants this line 4 reported in this case.
In your case, what is likely that either your IRA distribution or your pension/annuity is 100% taxable, so the variable for the total distribution is set to blank (as before) but now it's added to the other distribution which was not 100% taxable so the total distribution is printed. The result is that the 4a amount contains only the IRA or pension/annuity amounts but 4b contains both amounts.
Does this describe your situation?