Well it is not a phenomenon that I have noticed but I am more than happy to praise your powers of observation. I am a strong advocate of taking a few minutes every time I go to my allotment to simply look at things. It is worth the time spent as so often you detect something interesting.
Just one example this season, I have noticed that some of my seed packets seemed to contain more than one variety. My first row of beetroot were Boltardy. Nothing obvious from the seeds themselves but harvesting there are unmistakable signs that some had a different bulb shape, slightly different leaf shape and paler colour and ifmore distinctive rings when cooked. Also my sugar snap peas looked to have some slight variation in wrinkliness when they were sown and when grown two clearly different pod shapes were apparent, one type very flat initially. I strongly suspect the same with my celeriac as they have differences in leaf colour and habit. No evidence for any other types or crop.
These were all seeds from Kings and I'll be watching closely next season. They are keenly priced and I will give them another chance, but may switch if it happens again. Dealing with so many varieties and so many crops, the odd interloper is to be expected, but a third of the packet?