I agree with you. I have a similar case and I have provided 2 covering letters. One for explaining all the supporting documents and the other one specific to previous earning. The previous earning cover letter letter is as below:
Dear Sir/Madam,
RE: Explanation of Salary Statement and previous earnings
Kindly find an explanation below for the Gross Salary claimed as £ XX:
1. P60 contains the Taxable Pay not the Gross Pay for the year 2006 - 07
2. As per March 2007 Salary Slip:
i. Taxable Pay = £ XXXX
ii. Gross Pay = £ XXXX
3. Kindly refer to April 2006 pay slip for clarification.
i. The deviation is due to the fact that "-------"
ii.I have mentioned the detail breakup of Cumulative Taxable Amount Cumulative Gross Amount in the “Income for past fourteen months“ grid.
The documents provided are:
 Salary Slips: This shows the Gross Pay, Net Pay and Taxable Pay for the period March 2006 till May 2007
 P60: This shows the Taxable Pay for the year 2006 – 07
 Bank Statements: This shows the Net Pay credited each month from 14th March 2006 till 23rd June 2007
 Salary Certificate: This shows the Gross Pay from the employer, Employer's Name
I trust the information I have provided is satisfactory for granting me XX points under previous earnings claim. However if you have any questions or queries or require additional information, please do not hesitate to contact me on XXX or alternatively via email
XX@XX.com.
Many thanks for your time and effort in advance.
Yours Sincerely
YOUR NAME
Hope it helps!!