I closed my retail pro lab in 2011 and moved my mini lab to my home. What I have learned from the Noritsu Techs that serviced my machines is that the big box stores run their replenishment rates very low to save on chemistry costs, For example; lets say for every roll of film I process my lab will draw in 10 ml of fresh chemistry to keep the developer strength at the proper level. The big box store will only replenish 2ml by comparison ( these are not actual numbers) . The point is after a very short time they are running weak developer, which adversely effects contrast and saturation then eventually density runs amuck as well. Because film is somewhat forgiving, if you nailed the exposure then the printer can still make you a semi-decent print but your Negative is not properly developed and will have an inherent weakness when it comes time to print. If you were of 2 or more stops the print doesn't look all that good because the neg is weak.
The other lesson I learned from the technicians is that most of the big box stores to not run daily chemical test strips and plot the results so they don't know where the strength of their chemicals are at any given time and when the developer is on the verge of exhaustion. They usually ruin a roll or two before they figure it out and the customer usually accepts the fact that it was something they did wrong. Strange but true.