I think they system is confusing.
The difference is very much as you said:-
Justice intended to kill
In order to be convicted of manslaughter the prosecution needs to show that the defendant did something illegal and also intended to cause the victim at least some harm.
In this case, according to the link above, the victim withdrew his complaint and said that he fell down while drunk and not because she knocked him down.
If she had knocked him down then there is a good chance that she would have been found guilty of manslaughter.
As it is, she was convicted of ABH. She was sentenced to 16 months and I would guess that she probably pleaded guilty.
When a person pleads guilty at an early stage they get a one third reduction in their sentence. So the judge would have given her two years which after the one third reduction leaves just 16 months.
When it comes to ABH that is very near the top of the sentencing guidelines which go from a community order up to a starting point of 18 months. The maximum sentence is 3 years.
The starting point for the highest category which involves greater harm (serious injury must normally be present) and higher culpability is 18 months.
So the judge appears to have gone further than this and given her 2 years but then is required to deduct one third for an early plea.