Tax You Owe - calculations

[R1200GS]

Could someone please help me understand how HMRC have calculated an underpayment of Income Tax which is described on a recent tax code notice, but doesn't make much sense to me?
I have apparently underpaid IT for the period April 2017/2018. This, I am told by the explanatory notes is £85. HMRC are going to adjust my Tax Code by reducing my tax-free allowance by £426. This I understand, (I think!) 426 x 20% = 85.20
However, in the same Tax Code letter, HMRC calculate that I am going to underpay IT this financial year (2018/2019) by £196. They have stated that they will reduce my tax-free amount by £1334 to collect this amount. However, 1334 x 20% does not equal 196, but 267.

My second question relates to the HMRC IT web site. I have managed to look at my IT page on GOV.UK and have noticed that the figures are not the same as the Tax Code letter. The first underpayment figure is the same (£426 is used to collect £85.20), but the second underpayment figures are different. They tell me that £1428 is going to be taken off my tax-free amount to collect £173.80. This is baffling as the amount they need to collect is smaller, but the amount they intend to take off my tax-free amount is bigger! Putting aside the question I have over why I am underpaying IT, why are the calculations different and how should I calculate the amounts correctly (or have HMRC got it wrong)?

Thank you in advance.

[D&C]

It is likely to be different because you are really comparing apples and pears

The £85 is being collected over the whole year so (6 April to 5 April)

The £196 is being collected from whenever your code was changed to account for this (August 2018?) to 5 April. So a bigger deduction is required as there are less months where this code will be used, probably 7 or 8 for you on this occaison.

You should really ignore the different code on your personal tax account. This will change every day* if you could be bothered to check it. But this is irrelevant, it is only the codes actually issued to you and your employer which matter.
* the underpayment restriction will be recalculated to take account of how many days/weeks/months there are till the end of the tax year. In between pay days it will increase but then most likely revert back to similar to the original figures after you are paid next (and your employer tells HMRC your updated year to date pay and tax deuction figures).

[JRG]

Re 2017/18 tax year:
HMRC should have given you a P800 form that shows their calculation.
Compare their figures against your P60 and any other income, and if they differ, advise HMRC so that they can correct their records.

Re 2018/19 tax year:
HMRC doesn’t know your annual income. It estimates figures and uses them to calculate your tax liability.
Check the figures on HMRC’s website and correct them if they are wrong.

Any change to the tax code applies to the whole tax year, unless HMRC sets it to monthly basis (e.g. 1000Lm1 instead of 1000L).
If your tax code goes down, you will pay more tax on your next payday to collect the underpaid tax to date.
If your tax code goes up, you will pay less tax on your next payday to refund the overpaid tax to date.

[R1200GS]

Thank you both for your very helpful advice.
Chris