Income tax- Pay more tax than you should?

[Letree]

I am a HSMP holder and try to know more UK tax.

I found PAYE tax calculator from HMRC wesite(it calculates your tax payable deducted from your salary) charges you more tax than you should pay.

For example, monthly salary: 4000, then total annual is 48,000,

Exemption allowance for 2007/2008 is 5225, so the total taxable income is 42,775.

Tax band: Tax rate Tax
2230 10% 223
32370(34600-223) 22% 7121.40
Above 32370 40% 3270 ( 42775-32370)* 40%


Total 10614.40
Monthly (/12) 884.53

So monthly tax for 4000 salary should be 884.53, however, if you use PAYE salary calculator, the monthly payment is 904.6, you pay 20 pounds more than you should pay each month...

Here is the PAYE tax calculator link, use the tax code this year( 522L), and 4000 monthly salary, you will get 904.6, and this is the amount your employer will deduct from your salary.

http://payecalculator.inlandrevenue.gov.uk/PAYE0.aspx

I also calculate the tax with other annual income using the calculator, e.g. 40,000 etc, and I found the amount from PAYE tax calculator is always 20 pounds higher than you should pay.

Does anyone know the logic from PAYE tax calculator?