The best way to manage variable monthly payslips is to apply under Category B
Category B encompasses circumstances for those having a variable income.
Under this category you must be in employment at the date of application and be in a 'non-salaried employment' or receiving a variable income and wish to be considered in this category rather than under Category A.
To be compliant under Category B you must meet requirements and show evidence in two parts
Firstly
Where you are in 'salaried employment' at the date of application, and earning the amount relied upon you can count the gross annual salary at the date of application towards the financial requirement.
Gross income from 'non-salaried employment' will be counted on the same basis as income from 'salaried employment' where you have been with the same employer, or earning the amount relied upon, for less than 6 months at the date of application.
('non-salaried employment' includes that paid at an hourly or other rate or paid an amount which varies according to the work undertaken.)
When you're in 'non-salaried employment', the level of gross annual income relied upon in the application can be no greater than the annual equivalent of your average gross monthly income from 'non-salaried employment' in the 6 months prior to the date of application, regardless of whether that employment was held throughout that period.
So, the calculation will include all non-salaried employment undertaken during that 6 month period, and will be calculated over that 6 month period regardless of how much of that period was actually spent in employment.
The calculation for this annualised average of 'non-salaried employment' in Category B is made like this:-
(Total gross income from all employment undertaken during the 6 month period, divided by 6) multiplied by 12 = Income from non-salaried employment that can be counted towards the financial requirement.
Secondly
You must ADDITIONALLY have received in the 12 months before the date of application that level of income required to meet the financial requirement.
Your P60 will be important for this evidence.
So the fact that one month payslip is lower will not cause a problem since the UKBA calculation is based on total income during the previous 6 months followed up with evidence of income over 12 months.
Hope that helps