Might depend how the prior year tax compares to current year withholding and estimated tax payments.

Go through the 2210 and worksheets carefully and you may find a difference in there somewhere.

client with $598 penalty tax liability $147285

client with $53 penalty tax liability $142098

And what were their 2021 Withholding/Estimated payments?

client 1 with $598 penalty had no withholding, self employed, estimated payments of $97124.

client 2 with $53 penalty had withholding of $1942 and self employment estimates of $45288

Print the 2210 worksheet for each of them and go through them side by side, particularly part III and see if you can see a pattern which illuminates your difference to you.

Maybe they paid estimates at different times during the year. And that compared to withholding, and compared to last year's tax liability

thanks for the input, return has been filed but i will continue to review for answers

pay dates for both clients are the same, 4-15-21, 6-15-21, 9-15-21, 12-31-21.