It used to be the case thatvegetable seeds were required to meet minimum standards by sample , and the Seeds Act 1920 contained a table of minimum germination. This was declared by seedsmen as a minimum . As you may expect, the larger the seed , the higher the percentage. If I remember correctly , in the range of 78-96% Until a few years ago, stockists of seeds bought on a "sale or return basis, the returned seeds were sampled, blended and repacketed for the coming year. I believe that flower seeds were not covered by the act but the returned seeds were similarly treated.
In my experience, it will be most significant for some varieties if they are stored incorrectly, or not put through the correct temperature cycle. TOO WARM CAN BE AS BAD AS TOO COOL.
Similarly, I believe that the biggest cause for failure is over watering. The seeds generally only need moisture sufficient to restore a living state to allow the germination process. It is all too easy to keep running a can over trays just because the surface looks dry. Also the classic seed compost may contain peat and humus products which act like sponges, good for root formation but wet for the initial phase. You could try "stretching" the compost you use with silver or sharp sand or perlite (vermiculite)and placing trays on free draining surfaces.
Another approach would be to sample the seed by taking ,say 10 seeds and placing them ondamp kitchen paper on your window cill. watch them daily and notice when they show signs of the radical(rootstem)emerging. If you get 8/10 the seed is OK