Site doesn’t show correct AdSense size…why?

Hi All,
I am just wondering why my sites are not showing the right size of AdSense advertisements.
My site shows a 300×250 ad size but I embedded a 336×280…I thought it will just change over time but it's not. It's like almost a year, maybe. I tried checking the codes, reinjecting it, checking it over again thru the browsers "view source" but still it's not showing the right size of the ad.
I appreciate any help.
Thank you so much!

See original post by manchingjp