While it may be true that there could be discrepancies on a few item, i doubt this is a deliberate move to defraud customers. I shop at Uchumi and even for a monthly shopping, my budget variance is negligible so it is not like every item has a discrepancy.
sometimes you find, the price indicated is may be for a different brand of the same item. eg, sometimes back i went to buy rice. I know i buy CIL at around 290 or so for 2 kgs. but where i found it, the price was around 255 or so.... and i was excited that they are giving discounts only for the counter to change and i raised concern. when we went back to the shelf, the price indicated was for another brand i think "guru" or something but it is still rice, 2 kgs but the attendants may be did not bother putting the CIL at its right place.
So, it does happen.