I'm thinking it may pertain more to the novice. Someone who is new to gardening will probably not offer the seeds optimal growing conditions. therefore not all of the seeds are likely to germinate. People forget to keep them moist, they use bad soil, forget to water after seedlings sprout, bad temps etc. So, they are likely to lose a lot of seeds/seedlings. SO if they plant 20 seeds where they want ten plants, they are more likely to get the results they wanted. Then the new gardener says "oh those seeds were so great, let me buy that same brand next year!" and the seed co. looks good.
But if the rookie plants ten, and only wants ten, but only is successful with 4 or 5 plants, they will never blame themselves for their gardening methods.....so it must be the seed company's fault for selling them crappy seeds.
I plant the seeds generously and find reasons every year to justify overcrowding. Which seems to be the rule of thumb around here based on other threads. lol