because the number ends with trzy.
(if the number ends with dwa(2), trzy(3), cztery(4)- numerals act like adjectives. (2,3,4,22, 23,24, 32,33,34,...102,103,104, 122,123,124...)
for all rest numbers, nouns act weird (nominative and accusative change to genitive)
Thank you for explaining. In Russian, nouns after 2,3, and 4 take genitive singular form, and nouns after 5 or above take genitive pleural form. I wonder if Polish is the same. I wish Polish Duo has more official teaching comments/explanations in each topic as in other languages.
It is not the same as Russian, but I guess the line between different cases is the same (2,3,4 vs 5,6,7,8,9)
Contributors are working on the Tips and Notes but it may take a while before anything new is added.
If you need to learn more about numerals I think this can help:
I have a question. Do you know whether such complex system has any practical reason?