The big difference is that bullying and burnout in the workplace are not accidents, whereas bumps, bruises and scrapes usually are, @Roseypurple. Children run around without looking where they are going and trip over, they catch themselves on things, they bump into each other or the staff.
Of course the staff should do what they can to prevent these accidents or lessen the impact of them - checking toys for sharp bits, trying to keep the floor clear of trip hazards, watching the children - but they cannot be wrapped in cotton wool. No child gets through childhood without some bruises and scrapes, unless they are never allowed to do anything - and that would be more damaging than a skinned knee, imo.
I raised three boys to adulthood, and I can assure you that they had their fair share of bumps, bruises, scrapes and fractures, and on plenty of occasions their dad and I could not have told you what had happened in minute detail, because we couldn’t have our eyes on three of them at the same time. Nursery workers are trying to supervise a number of children, and they can’t always be watching every child, so the explanation you got is probably the most detailed the staff member could make it. Better they are honest than that they make up details they don’t actually know.
Picture it - your hand is by your side and a child runs up to you and runs into your hand, while you are watching another child. You don’t see the incident, but feel the child run into you and see a small scratch on their face - that is all the detail the staff member will be able to give you.