It is that some are vulnerable to crossing - as I understand it. Some varieties have such tight petals that they are all but completely closed to outside pollen.
I began growing my grandmother's tomato and saving seed from it about 25 years ago. When I started doing that, I never gave cross-pollination a thought. Those plants have never been alone in the garden. At first, there were a couple more varieties. About 10 years ago, I began to grow about 60 plants and as many as 30 varieties each year. I have never noticed a change in the plants grown from Grandmother's tomato seed.
There are usually multi-year seed stored from several dozen varieties on my shelves. I'm not selling it. If plants turn up in my garden that are different from what I expected, I can fall back on a previous year's seed or buy new (except for Grandmother's
I was once given seed for Kellogg's Breakfast. One of the plants had red fruit that ripened early! I saved seed. The next season, half of those plants had red fruit and half had yellow, showing that their parent was indeed a cross. I saved seed from the red fruit and am anxiously awaiting fruit to ripen on this season's 4 plants (grandchildren
). I hope I don't lose the earliness while stabilizing the fruit color. Fun!