Not CalPERS but I looked at the 1099-R form on IRS website and on Box 5, it says Employee contributions/Designated Roth contributions or insurance premiums.
Here's what it says on the back of the form. Did you by any chance purchase a larger pension with after tax money?
Box 5. Generally, this shows the employee’s investment in the contract (after-tax contributions), if any, recovered tax free this year; the portion that is your basis in a designated Roth account; the part of premiums paid on commercial annuities or insurance contracts recovered tax free; or the nontaxable part of a charitable gift annuity. This box does not show any IRA contributions. If the amount shown is your basis in a designated Roth account, the year you first made contributions to that account may be entered in box 11.