Why PCB is different every month?

PCB will be calculated for one annum every month and divided by the remaining balance of the months to get the current month's tax.

This calculation is affected by a few factors as below :

a) Accumulated Income

Check the accumulated income as the PCB only will be calculated when the chargeable income exceeds RM37,333 after the EPF deduction.

https://www.hasil.gov.my/en/individual/introduction-individual-income-tax/when-is-taxable/

b) Residency status

Kindly check below article link below to learn more about Income Tax Resident status:

How to decide Income Tax Resident Status for employee? : Knowledge Base (freshdesk.com) 

c) Marriage status ( Tax Category)

The marriage status of an employee will affect the tax calculation

https://www.hasil.gov.my/en/individual/individual-life-cycle/how-to-declare-income/type-of-assessment/

d) Number of children

Based on the number of children and their educational status, the tax relief will be determined. Check the below article for more details:

Number of Dependent Children (Auto VS Manual) in Employee Profile : Knowledge Base (freshdesk.com) 

e) Types of Fixed / Additional Earnings

You may check it out what types of earnings are subjective to the tax as below :

Terms in Tax Deduction ( PCB / MTD) : Knowledge Base (freshdesk.com)

f) Tax Relief

Tax Relief plays a major part in reducing the taxed amount  in a month

Terms in Tax Deduction ( PCB / MTD) : Knowledge Base (freshdesk.com)

Why SOCSO Regarded As Relief in The EA Form? : Knowledge Base (freshdesk.com)  

g) Statutory Deduction ( KWSP / SOCSO + EIS) 

The statutory contribution plays a significant part in affecting the calculation of the tax amount 

Why SOCSO Regarded As Relief in The EA Form? : Knowledge Base (freshdesk.com) 

PCB formula is designated in a way to calculate chargeable income every month in case there are any salary changes throughout the months, and deducted with tax relief ( if any) and accumulated statutory deduction.

Example :

P ( Chargeable Income) = { Current Month Salary ( RM 4000 - RM 298 ( EPF) } + { Estimated salary for upcoming months ( RM 4000 - RM 298 x 11 months) }  + { tax relief ( RM 9000: self deduction) + RM 2000 ( 1 child) + RM 29.55 ( SOCSO + EIS) }

So the above formula has 3 parts, where it takes account of the,

current month's salary;

estimated salary for balance months, and ;

allowable deduction + tax reliefs.

Since the additional deduction of KWSP and SOCSO +EIS changes every month based on the earnings ( OT, allowances, etc) of the particular month, 

thus its accumulation will reduce/increase the chargeable income that determines the tax bracket.

To learn more about tax brackets, click the below link :

https://www.hasil.gov.my/en/individual/individual-life-cycle/how-to-declare-income/tax-rate/