I've also noticed this and would like to know more. I feel like this didn't happen on HN 5 years ago, but it does now.
I always wondered if maybe it keeps track of who has voted on your submissions in the past, and only gives you karma if the voter hasn't voted on you before.
Not sure whether it's influenced by your existing karma. My perception (with sample size of one) is that when my karma was lower I accrued at closer to 1:1, whereas now definitely not accruing at 1:1.
I always wondered if maybe it keeps track of who has voted on your submissions in the past, and only gives you karma if the voter hasn't voted on you before.