PAYE is not a flat percentage of what you earned this month. Your employer takes the month's pay, projects it across a full year, works out the annual tax on that projection, and divides back down. When every month is identical the answer is identical too.
The moment a month is not typical, the projection changes. Overtime, a commission payment or a leave payout in March gets annualised as if you will receive it every month for a year, which can push the projection into a higher bracket and take a disproportionate amount of tax off that one payment. It corrects itself over the year, and any genuine overpayment comes back on assessment.
A few other things move the number without your salary changing. Turning 65 or 75 changes your rebate. Starting or stopping a medical aid changes your tax credits. Beginning a retirement annuity deduction reduces your taxable income. And in March, when the tax year restarts, new brackets and rebates take effect.
If the change is large and none of those explain it, compare two payslips line by line before raising it. The cause is usually a deduction that appeared or disappeared rather than the tax calculation itself.