I believe that this depends on what type of job you have and when certain types of your wages and any bonus/comission are paid.
Example:
Someone who is paid overtime above contracted hours and bonusses one month in arrears would, I think, have different figures on P60 to their month 12 payslip. As overtime and bonusses would appear on the following month's payslip. Similarly, the previous month 1 payslip would contain bonusses and overtime from the previous month.
However please correct me someone if I am wrong.
Hope this makes sense.