yeah. Numbers are the culprit. If there was no number, the sentence would be
Nesu poslední vajíčka.
If you brought just 2, 3 or 4 eggs it still would be Nesu poslední 3 (2,4) vajíčka.
But you are bringing a dozen. That is more than five and everything changes from five up.
Read on it here https://www.duolingo.com/skill/cs/Numbers-2