Venkat,
you need to have Admin Rights to get page_fans for any page as only Admin Access Token can return you that result. What you're doing in Graph Explorer is using your Access Token to view insights so only limited fields are visible to you
The Insights that you're viewing is actually Public Data, that can be fetched for any page, but if you're admin then you would be viewing different data which is private to the admin users only. (only data you see in insights is the public data, that can only be viewed for any page)
Docs:
https://developers.facebook.com/docs/insights/
https://developers.facebook.com/docs/graph-api/reference/insights/