As Dinafem is a legitimate business and is also the owner of some excellent and in some cases original (The Cheese, White Widow among others) genetics you can guarantee that the quality of the seeds they produce will be of the highest standard. BUT to ensure they maintain this standard they go through quite a stringent selection process. This means that when they batch test them, any batch that falls below 90% germination but still above 70% is classed as a "Second". Like any natural plant you will get some variance in the genetic make-up but the strains they breed are done on such a massive scale they are able to even out the standard, remove any mutated variations and keep the genetics true.
Some of their newer strains are still being "bedded in" - Critical Jack and Haze Automatic being the two most notable - which means that they are slightly more prone to not breed quite as true as some of their other strains, but having said that they are still very high quality.
All of this information is available from Dinafem so if in doubt just ask them yourselves! No I do not work for Dinafem but I have been dealing with them for quite some time and have met them a number of times at Cannabis Expo's