d2r classic (Diablo 2)

I play single xc 8 ppl on a normal beaver. and here's what I noticed. running in the second act. and there already got a bird with physical immunity. I was surprised. but then in act 3 I got unique with immunity to light. I haven't seen immunities in act 4 yet. question: where do immunities come from on such complexity? it's normal! there is no such nonsense in LOD