The late ones are those particular seeds that required a cooler temperature to germinate (as compared to the rest of the lot). They were just laying there, waiting for cooler media temperature. I've had it happen when seed was started normally at the typical 65'F to 70''F, where only about half came up as usual and the remainder didn't come up until late Oct./early Nov. By then the nighttime lows were often below freezing but I think that's what it took. I did the same as you, I separated the two groups, etc. But one thing I didn't do and I should have, was follow up and compare the differences of the two groups as mature plants. Maybe that's something you might want to do since you have the opportunity.
