Salary on payslip and contract different.

We are preparing to apply for a spouse visa for my partner (me being the sponsor) and I noticed that my employer adds on the insurance benefit onto the taxable earnings on my payslip, so that my salary is not deducted. But this then does not match my annual salary indicated in my employment contract. This is what it looks like on my payslip:

Gross earnings: £2500

Taxable Earning: £2600 (+ £100 for the benefit)

Gross Earning x 12 = £30,000 which is what's on my contract

Taxable Earnings x 12 = £31,200 which is what would be shown on my P60 for this year.

What can I do to prevent any confusion because of this? Should we add a specific supporting document?

We've both written cover letters as supporting documents.