A form 1099-R reports the pension distribution in box 1 and the taxable portion of the pension distribution in box 2a. The difference is because you contributed some portion to the pension fund and the IRS presumes that you pay back a portion of that contribution on each year's income tax return.
The amount previously recovered is how much of the pension contribution you are considered to have paid back through the prior year's income tax returns.
You may be computing the taxable portion of the pension distribution in box 2a. You are likely using the simplified method of computing the taxable portion.