I have posted about this before. It's a bit long but I will try to be short and I'm happy to give more details.
Chester was a very small intensive care unit. It had only three intensive care cots. The other babies needed less intensive nursing.
Because Chester was so small, on some nights no intensive care nurses were needed at all. You always needed two senior nurses and two more nurses, who could be junior.
The nights when babies were most likely to die were the nights when more of them were intensive care cases.
If you had two intensive care cases, you needed two senior nurses plus one other nurse (senior or junior) with a special intensive care qualification.
Only two junior nurses had intensive care qualifications. Lucy Letby was one, and worked night shifts. She was also very willing to be moved to shifts where her qualification was needed, and to work extra shifts.
So, you're Lucy Letby's manager. You are trying to staff a night shift. There are two or more children in intensive care. You get two of your senior nurses in, out of about sixteen. Those sixteen will see a lot of deaths between them, but it will be spread out between them. The next person you call is Lucy Letby. If she is available, that's cheaper than a third senior nurse. And she agrees again and again.
At night, the unit is consistently understaffed. The consultants at more specialist hospitals aren't available to give advice remotely. The transport service and its expert team takes hours to arrange. The unit's own consultants can take up to half an hour to attend - it's meant to be five minutes by day. At night, babies are more likely to die. Especially the most fragile intensive care babies. And that's when Lucy Letby is most likely to be working, and that's why she saw so many babies die on her shift. Because she was the cheapest qualified nurse available to work when babies were most likely to die.