How do you know they use different scalers? AFAIK, the only manufacturer that uses different scalers in their models is Sony (ie: the different bravia engine flavors)
Be careful with that kind of blanket statement. When I bought my Olevia 542i, they were using one display chipset with their 1xx and 3xx series, another with the 5xx and a very high end one in their 7xx line. The panels for each size were generally the same too. Not to mention some lines actually had different chipsets once you got past a certain size. There are a number of display chipset makers out there catering to various price points.