For a while I was observing unusual behaviour of my 336 by 280 Ad units where smaller sized ads were get delivered in the size of 300 by 250 units.
I even report this issue to Google AdSense Support Team but received no reply & currently they were not accepting any new support request but any how I got the solution that I wanted.
I got the solution to the problem why smaller unit ads were get delivered in bigger size ad units & there by ruining my ‘Website Design Layouts’
Here is what Google Says about that:
(Source: Adsense Resource Tab)