Graham, there is a third missing interpretation.
Cyclists accidents are not getting reported as they are protecting the head against injury up until a point, not dissimilar to the second point but another way of looking at it. It's taking more to cause that injury that is being reported.
If less injuries were being reported because of helmet use then the head injury numbers should drop. The fact that they remain roughly the same would require that the diminishing number of non helmet users to dramatically increase their number of head injuries at the same time.
(possibly related to point one I don't feel it is, or there are more people riding in more dangerous places i.e. more people just riding on the roads which are getting more dangerous (more cars, increased public transport, cars parked on roads and pavements, cars are larger taking up more room) in general causing the chances of going beyond a helmet capabilities.
It does not mean a helmet is not working and reducing severity or not saving lives.
It also depends how you phrase it....
The number head injury in most areas of life is slowly reducing and has been doing so for decades. This does not correlate with helmet use, and pedestrians and car users do not wear helmets anyway. This ongoing reduction is probably due to hundreds of minor health and safety improvements. Car design, street design, driver awareness, better lighting, etc, etc.
It would be interesting to have some more recent data on the the last graphs, 15 years have past since to see if the trend has just stabilized and also to see what type of accident or if other influences like speed limits, car rules, location are involved. Always the problem with small data sets (the first of the three shows nothing really)
I hope you get my point in my jumble of words
This data must be somewhere? And It would provide many answers.
In particular, data on the ratio of rotational versus linear momentum brain injuries would be conclusive in telling us how effective current helmet designs are.
9/16" Suntour BMX MP-1000 pedals / 56 tooth TA 6-bolt chainring.(I'm not bothered if the teeth are worn or damaged)