Interesting. I've never heard of hyper focus in relation to PTSD as a common symptom. But certainly, if a natural coping mechanism to inattention is hyper focus, people with PTSD would tend to hyper focus. I also think it might have to do with the neural pathways that affect both.
ADHD is an issue of under stimulation of the prefrontal cortex (the place of concentration and focus). So when stimulants increase the response of the brain to things around it, ADHD is treated. PTSD is an issue of overstimulation in part in the same areas. It is the same as what happens when an ADHD kid get stressed, except chronically and obviously for different reasons. The prefrontal cortex is flooded with adrenaline, causing a mental shutdown. Your logical thinking shuts down. With PTSD, this trigger is accompanied with mental recall of trauma, etc... With ADHD, a kid might just avoid whatever is causing the stress (homework, school, etc...) and get lost in various addictions (video games or TV is common for young kids, drugs/alcohol for teens and adults). They really are extremely similar in terms of brain activity.
I say all that to say, it makes sense that there would be such a huge overlap. There are certain things that are different (the involvement of memories for PTSD is the biggest, along with the opposite effects of overstimulation versus under stimulation) that cause the differences in hyperarousal and inattentiveness. But overall, they have some major similarities.