"A commission headed by Lew Allen , director of the Jet Propulsion Laboratory , was established to determine how the error could have arisen. The Allen Commission found that the main null corrector , a testing device used to achieve a properly shaped non-spherical mirror, had been incorrectly assembled?one lens was out of position by 1.3 mm.^[60] During the initial grinding and polishing of the mirror, Perkin-Elmer analyzed its surface with two conventional null correctors. However, for the final manufacturing step (figuring ), they switched to a custom-built null corrector, designed explicitly to meet very strict tolerances. Ironically, this device was assembled incorrectly, resulting in an extremely precise (but wrong) shape for the mirror. There was one final opportunity to catch the error, since a few of the final tests needed to use conventional null correctors for various technical reasons. These tests correctly indicated spherical aberration . However, the company ignored these results, as it believed they were less accurate than the primary device which reported that the mirror was perfectly figured."
So they were well able to predict the lens. They simply didn't test it properly. Or rather they used faulty test equipment