Its not uncommon. The seeds are mature in the green fruits by the time the color breaks, but there are gemination inhibitors in the gel that prevent in-fruit germination, and in cooler climates inhibit germination long enough so that cold weather dormancy can set in. In the tropics this is more likely a dry season thing - you don't want your seeds to sprout at the end of the growing season right at the beginning of the dry season.
There is both random and environmentally induced developmental variability, and some of your seeds just didn't get enough germination inhibitors in those few seeds. I just had some seed germinate out of a catfaced fruit during fermentation-seed extraction only two days after they were out of the fruit.