I speculate that there was some goal-chasing. The PO had a prosecutions team. They need to earn their supper, management declares, here's a target for prosecutions and, more importantly, successful prosecutions.
After that, it snowballs, once they'd rolled into this goldmine of prosecutorial success, they couldn't just fess up that the convictions they'd achieved were obtained incorrectly (and illegally), that they'd conspired to obfuscate and hide evidence to make them, so they kept at it. Even if wasn't immediately obvious something was wrong, at some point they must have looked at the numbers and realised. And they kept at it.
Frankly, if she didn't know (and I don't believe it for one second), she's guilty of criminal ignorance. Yet, she got a CBE and year-after-year bonuses.