Just a quick look around reveals that "Big Beef" can refer to a hybrid, OR an open pollinated (true) variety. So what matters is which of these your original plants were. Where did you get them? And where did the
seed ultimately come from (that produced this year's plants)?
If your plants were not labeled "rare" or "heirloom" my bet would be they came from hybrid
seed. If this is the case, then you are correct-- the
seeds you have may not be "true". Remember Intro to Biology? If you cross two hybrid individuals, you'll get some hybrids, some like the "mother" (which contributed to the hybrid) and some like the "father": Aa X Aa --> AA, Aa, aa. It's a little more complex than that, but you get the idea.
Of course, the term "hybrid" is also being used to refer to genetically modified
seed.
If you are interested in
growing heirloom tomatoes, I like the selections at Botanical Interests, and at New Hope
seed Co. IMHO-- heirloom tomatoes beat the bejeepers out of all the rest. And contrary to what some folks insist, I have had fewer pest problems with heirlooms than with the "new & improved" varieties.