Employees owe HMRC tax P800

[Spidersleg]

One of my employees has received a P800 and owes HMRC £50.20, but what seems strange is that his wife who works for someone else has had the same letter with the same £50.20 underpayment even though their incomes were very different for 20/21

So my question is, how can it be they owe the same amount if they earn very differing wages? Also to add 3 other employees at the wifes' work have had the letter with the same £50.20 figure.

My first thought was scam! but they received letters and they don't have to do anything as HMRC will begin taking money next April.