Beginner question on curation ('upvoting') rewards

I joined steem recently (10+ days ago) .. I upvoted a few steemit posts that I liked/found interesting (upvoted them 30+ minutes after they were published,, as per this post that I found on google: https://steemit.com/toolkit/@sndbox/earn-steem-by-upvoting-how-curation-rewards-work-on-steemit ...),, 7+ days have passed, and I got curation rewards of 0.001 SP for only one of the posts that I up-voted that day,, which I don't quite understand how it "stands out" among other posts that were also upvoted that same day...

1. Is there a minimum that my upvote has to 'earn' so that curation rewards would be distributed to me?
2. How do I determine by myself mathematically, the 'curation' (e.g. 'upvoting') payout, after 7 days has passed e.g. why this specific post has 'paid': https://steemit.com/hive-174578/@davidesimoncini/winter-wood-in-black-and-white-8-photos and why that one 'has not': https://steemit.com/hive-174578/@tarazkp/do-you-need-steem-to-survive ?