Exactly
You'd never be expected to check everything. You would normally do a sample check where the auditee has a large data set (in this case, the 600 inmates at Fripps).
If I decide to audit a 5% sample, then I'm saying that I am 95% confident that the auditee's management system is compliant and I accept a 5% risk that the sample might not be fully representative of the overall batch. You can always adjust your sample size depending on other observations & variables.
I looked up APHA - do you supersleuths know what kind of audit they did on Fripps? All I could find was this -
https://www.gov.uk/guidance/national-feed-audit-inspection#who-gets-inspected which is a 1 hour check on feed receipts & storage?! That sounds super basic to me.