When you retired, an actuarial calculation was made to determine the number of units you would be paid as part of your retirement allowance. The calculation was based on several factors, including the number of units you had at retirement, your retirement elections, and life expectancy. (If you chose to receive your TDA funds as an annuity, a similar calculation was made.)
Please keep in mind that the number of units you will be paid will normally remain constant from month to month unless you transfer funds to or from a variable-return Passport Fund.