I'm thinking the culture has a lot to do with the lines. With just a couple items sold in each store, you spend all your time going from one store to the next. They shop daily for food whereas I may go to the grocery store once or twice a month so I'm in only a couple lines a month rather than several lines every day. Your co-worker's mom wasn't freaking over the abundance of food but that it was all in one place.
When I see things happening in a communist country that don't happen in ours, the first explanation I look to is that difference.
If I had to guess, I would think that the supply of any given item is distributed to the "retailer" who offered the biggest bribe. This will vary depending upon how hungry the retailer gets. If he gets hungry enough, then the next item available is the one he is going to sell.
If you see retailers with the same products over a lengthy period of time, then you may just be seeing that a "long term contract" exists between that briber and the bribee.
The average Soviet bureaucrat may not want too many people to know that he will divert goods for cash. Minimizing the number of retailers with which he deals may be a security matter.
A similar situation existed when I was in the service. There were certain individuals with what was called "property book authority". They were authorized to sign for transfers of property over which they had control.
If you knew who had property book authority over galvanized pipe, then you knew someone who could supply you with galvanized pipe for the right price, whether the higher commanders agreed that you should get such property or not.