The SSP terms allow Delta access to all employee records, not just PRIA. Potentially, more available information will statistically eliminate more applicants. Combine that aspect with the lack of primary screening given to OFS applicants (because eligibility is based on employment only), it is not unexpected to see a lower pass rate. Endeavor staffing difficulties may also be playing a role, to some extent.
Whatever the early perceptions or expectations, no one at Endeavor thinks the SSP is a "gimme". For comparison: Endeavor management claims the chances for a Delta applicant to receive a CJO to be at 6%, overall. YMMV