Marginal slabs, not a single rate
Each slab applies only to the income inside it. If a slab begins at a certain annual figure, the higher rate touches only the rupees above that figure, not your whole salary.
That means the effective rate you actually pay is always lower than your top slab rate. Comparing effective rates, not slab labels, is the only meaningful way to judge a change in your take-home pay.
Why the deduction is monthly
Employers withhold tax each month against your projected annual salary and deposit it with the FBR. If your pay changes mid-year, the projection changes and the monthly deduction adjusts, sometimes sharply.
Bonuses are typically taxed in the month they are paid using the same annual projection, which is why a bonus month can look over-taxed and later months slightly lighter.
Filer status is separate and worth having
Filing a return does not change your salary slab, but it changes withholding rates on many other transactions: property, vehicles, banking and utilities among them.
Salaried people often assume employer withholding is the same as filing. It is not. If you never file a return, you remain a non-filer and pay higher withholding elsewhere despite tax already being deducted at source.
Checking your payslip
Multiply the monthly deduction by twelve and compare it with what a slab calculation on your annual package produces. Small differences are normal because of allowances and timing; large ones deserve a question to payroll.
Keep your annual salary certificate. It is what you need at filing time, and it is the document that proves tax was already paid on your behalf.