Many people have a problem with high cholesterol despite a low- fat diet so unfortunately this may be something that you have to use statins for, and there may be no obvious underlying factor that can be identified.
It sounds like you are doing everything you can to look after your health and this is really good, so continuing with statins will help protect your cardiovascular system long term.
Although you may not have a family history of high cholesterol, it is possible that previous generations passed away from other problems before the cholesterol became a problem. Other clues are family members that had angina or heart attack at a relatively young age (under age 55)
Other 'hidden' sources of cholesterol include eggs, cheese and cakes/biscuits but if you are having a healthy diet then I expect these have been excluded too
Long term use of statins is becoming very common, and as long as your doctor continues to monitor liver function and you report any new muscle pain then you should have no significant problems.