There are several possible explanations. For example:

• Your first payment may contain retroactive funds, so subsequent payments may be smaller than your first payment.
• Deductions for insurance and union dues may have begun (usually in the first few payments).
• If you elect to change the amount of federal taxes withheld from your payments, the amount of your payments would be different after your federal withholding election is implemented.
• If some of your funds are invested in the variable-return Passport Funds, the return on those investments would fluctuate from month to month.
• A revision of your retirement allowance may have been implemented, in which case you would receive a revised Benefits Letter.