Almost everything in the supermarket business is in the hands of two Thai families. So there's NO REAL competition.
CP Group: Makro, Lotus, 7/11, CP shops, ...
https://www.cpgroupglobal.com/en/about-cp/business-lines
Central Group: Central Department Store, Tops, Big C, Go Wholesale, Thaiwatsadu, PowerBuy, ....
https://www.centralgroup.com/en/home
So I believe it will never give a price fight between them.
And they do it the same as the Supermarket chains in EU & USA. If a holiday come the prices gone up.