True F1 seeds grown from primarily homozygous parents are very much a "strain." Of course, none of the plants we grow are actual strains in the biological sense, they are all just "varieties." A variety of something that grows consistently. True F1 seeds as they are advertising them will grow even more consistently and with greater vigor than inbred F5 seeds. It's just a fact of genetics. However, the key is that the parents are themselves inbred to the point of homozygous dominance or recessive on most alleles. In fact it usually takes two parents that are almost sickly from being so inbred, and that themselves express very very different characteristics. Simply combining two strains might make a F1 cross, but it's not the type of F1 cross they are talking about here. This approach has revolutionized commercial farming. For instance, the vast majority of corn you eat is F1 corn, because it grows faster, taller, stronger, more past resistant, and with more consistency which is better for commercial machinery. Also, you can't breed an F1 with an F1 and result in any consistency because, by definition, he will be mixing up a bunch of heterozygous alleles. This has been good for the corn seed companies, for instance, because farmers can't then just plant the seeds they grow. They need the F1. There is nothing about this process that is genetically modified or tricky. Pull out your bio 101 notes from college and sketch out the Punnett square to check it out. In most plants, there is a mixture of alleles, so if you had two parents and one has Ab and the other AA (one heterozygous, one homozygous dominant), then 50 percent of babies will be AA and 50 prevent Ab. What happens when you cross AA with bb? Every baby plant has Ab, perfect consistency. If most alleles from the parents are either dominant or recessive due too heavy inbreeding, then the baby plants will be highly consistent.
