Fixation on certain things and/or 'special' interests are one of the diagnostic criteria of asperger's. It's part of the black and white, rigid thinking. Aspies have trouble with shades of grey. So it's normal for them to have either no interest in something or to be obsessed by it, rarely anything in between. Thsi can often be the root cause of some behaviour problems for parents. Mum says come and empty the dishwasher, apie brain can't process switching from playing World of Warcraft, or arranging Barbies, or reading a certain book (example special interests) to something so boring. They also can't grasp that others don't see the importance of what they are doing. Result, screaming tantrums and anger.
It's not all bad though, if you can channel the interests and support them, within certain boundaries, you end up with an aspie at their best. Absolutely brilliant in a way others can't even dream of. My husband also has AS, his special interest was chemicals. The stories he tells of what he just to get up to in his mothers garden shed would make your toes curl. How she managed to stop herself killing him, I'll never know. He's now a professor of nuclear chemistry. My daughter has been obsessed with the holocaust and genocide for years, that's why she is so driven to teaching and working in Africa.
When I went through the AS assessment myself I scored incredibly high on the OCD tests. The doctor said that this would be a serious concern under normal circumstances, but for an aspie, it's to be expected. Aspies cannot make sense of the world around them so they create order that makes sense to them. Doing things in a certain order, routines, lists, lists of lists. It is very OCD like, but it isn't OCD.