If you could provide those figures by mile or by time spent in the activity it will make it a lot easier to answer. For myself, I have bashed my head once in a car (I was wearing a helmet), a few times whilst a pedestrian (none of which as an adult) and about 10-15 times as a cyclist. Per mile for me cycling is probably the most dangerous.
No need to provide by mile....
These figures are derived from the number of admissions due to head injury, requiring hospital admission and therefore a measure of the number of head injuries that could be prevented if helmets worked, and had been worn by these individuals
With the clear assumptions made it is a record of a head injury occurring and needing admission
Think of it as looking at a medicine, if you want to see the efficacy as a cure then you can only look at it's efficiency in those who have the disease.
No statistics will ever be absolutely accurate, and it is possible that other factors already affecting these figures such as air bags may have already reduced the number of car driver and passenger head injuries. Also you will note it includes motorcyclists who already wear helmets (and possibly some cyclists who were as well). the data (as with all data) is therefore limited, and it is up to you to make your own analysis and decide on its merit
It does leave us with a snapshot of the head injuries being admitted at the moment and under present circumstances
However what is evident is that when looking at head injuries that have occurred and needed hospital treatment is used as a baseline it is evident where the greatest improvements could be for the general population