Top Sidebar Banner Viewability at 30% - normal ?

Hello! I dont know which is the appropriate section for this,

Has anyone checked banner viewability for half page banners on the sidebar of ip4 ? 

I checked their viewability and its only 30% which is i think low for a banner above fold.   I am wondering if this is normal 

and perhaps if you have large % of mobile usage of site  where the banner will not show up (since the sidebar wont) 

but maybe the banner tag is beeing called  (is that true?)  and doesnt get an impression despite being "called" at every pageview? 

could that be a reason?  (dont know how else to describe it!)