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)
To ensure you are always getting the best results from your ad units, we recently introduced some updates to our ad serving process enabling our larger ad units to serve similar-sized display ads. We’ve already updated this for our 300×600 and 970×90 ad units and now we’ve extended this flexibility to our 336×280 ad unit, allowing it to serve a 300×250 ad when doing so will maximize the performance of that ad unit for the publisher.
With this change you may now occasionally see a 300×250 ad served in your 336×280 ad unit when that unit is opted in to showing image ads. 300×250 and 336×280 image ads will both compete for your 336×280 ad unit with the most competitive ad served thus maximizing the earnings on your ad unit. When a 300×250 image is served it will be centered in the 336×280 ad unit as illustrated below.
Hope it helps,
Thanks & Regards