All seeds contain nutrients that soil fungi and bacteria would like to use for themselves. The seed coat is a barrier protecting these nutrients from attack by outside organisms. If there were a lot of these fungi and bacteria in the soil where the seeds were planted, possibly the seeds with the injured seed coats were more strongly attacked than seeds with intact seed coats.
But some seeds also have very hard seed coats to insure that they don't all sprout at the same time. The hard coats prevent the entry of water, which is needed for germination. By not germinating all at the same, plants increase the probability that some of the seeds will germinate at a time when the weather is favorable, so that some seeds will survive to produce their own offspring.
Beans and peas have soft seed coats and a lot of nutrients. Their strategy is to germinate and grow fast. They have a lot of nutrients so that when they germinate, they can grow fast and perhaps outgrow infection by soil fungi and bacteria. The soft coats of these seeds let water in naturally when the seeds are planted. When you injured this soft seed coat by scratching it with sandpaper, you may have let in a lot of soil fungi and bacteria, which contaminated and used the nutrients inside the seed coat, and in this way prevented germination.
Had you used instead seeds with hard seed coats, like alfalfa, clover, or many of the common weed plants from farmers fields, the experiments probably would have shown that the seeds that you 'sandpapered' would have germinated faster than your 'controls', the seeds that you didn't scratch with the sandpaper. These kinds of seeds are usually small, and the seedlings grow slowly. But by not germinating all at the same time, it increases the chances that at least some of the seeds will germinate in good environments and survive.
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