Learn to price right!!!

I just want to know why is it that regularly I make purchases at pick n pay I have to memorize prices and check my till slip to check that I’ve been charged CORRECTLY. This is very inconvenient as I have to get the supervisor called or go wait by the info. desk for them to fix it... I’ve now began taking photos of the goods n keeping receipts... have two so far... one day after the other from pick n pay caversham for soy milk charged R26 for R17.99 priced good and discounted sweet corn 4’s at pinecrest pick n pay... promoted at two packs for R35 and was charged normal price of R22.99 x2 .. so like R45 some odd. This is so popular in retail... is it incompetency of ones job? Is it laziness to input correct changes on the system? Is it caused by lack of ethics of the company as a whole deliberately doing so , thinking we won’t notice? As much as it’s corrected once the supervisor comes to assist , but there’s no trade off for me being inconvenienced each time.