Inconsistent result with GetUserEffectivePermissions API

We are trying to check permission of the current user from our app, using CSOM function GetUserEffectivePermissions. The result returned is inconsistent where we can sometimes get the expected result (permission of the user) but sometimes not (Access Denied), even for a same user in a same site.

var basicPerms = ctx.Web.GetUserEffectivePermissions(clientContext.Web.CurrentUser.LoginName);

HTTP Request made:

http://pastebin.com/4fuJ3caz

HTTP response (with access denied message):

http://pastebin.com/UhTc2hX8

 Any ideas?