I'm not sure why they went so high since you were following the method on here. The only think I can think of is that perhaps the mature media had a lot of ammonia eaters but not many nitrite eaters and that ammonia was processed much faster than usual; so fast that there was no time for the nitrite eaters to start growing before the ammonia eaters removed all the ammonia. That's just sheer guesswork
Each 1 ppm ammonia is turned into 2.7 ppm nitrite, so keep the ammonia additions well spaced out and only 1 ppm doses. It doesn't take much ammonia to produce 15 ppm nitrite.
