Some inner pages as well as some product pages show supplemental in google, what's the reason for that and how can this be avoided?
This is because you have duplicate content. Change the content to unique, add your own heading and get some links to your page
Try to have unique content, titles and META tags across your pages. More importantly, attract deep links to these pages.
It means your pages or meta tags are too similar and therefore considered duplicates. It does not necessarily mean you have duplicate content but it means Google has a better version of those pages and will only go for those in the supplementary in times of need. The disadvantage to the webmaster is that, it may never appear in regular searches as it is considered second hand quality. To overcome this disadvantage, make your meta tags unique and avoid repeating some text throughout. Check the pages itself and remove the repeating content or change the font size to a smaller one if you have a large text for navigation.