IW Green Vouchers?

I'm just a bit confused about the drop rate for the Green Vouchers. Yesterday a friend and I did Infernal Walker for the first time, each of us getting 4 vouchers. We did it did it separately as well and still only got 4 vouchers. I've been doing it solo a bit this morning and I haven't gotten any more than 4, bur I see people on here getting up to 134 vouchers. Is the drop rate just random? I assume 4 being the highest drop rate and anything more being rare and harder to get? Just a bit confuzzled.