Fleataxi... that may well be true (and I've gotten mis-labeled and contaminated seeds before), but if so... it can be true of "heirloom" varieties, too.
And conversely, there are several varieties labeled "hybrid" which actually are stable OP types.
ANY OP variety is going to show some variation... that's typical and normal. In fact, it's *the* main reason hybrids became as popular as they did... a particular cross (which is what a hybrid is, a cross between two known OP varieties) will be extremely uniform... something you rarely see in OP varieties, heirloom or not.
And, in fact, that's a Good Thing... because simply "saving seeds" shouldn't be enough. Farmers and gardeners who really want to produce as much as possible, and as high quality crops as possible, need to select the best plants *in their garden or field* and only save seed from them. That need may well be why you are seeing what you feel is unacceptable variation in saved seeds... because doing the necessary "roguing" and selection on a commercial scale is difficult.
If you save seeds from a variety for 10 years, choosing the earliest or most productive, or latest to go to seed plants, you'll almost certainly have a new "strain" of that variety... and, in fact, if you are stringent enough on your selection criteria, you may end up with something different enough to almost be a new variety.
Summerthyme