Hi Hive account@karinlvira! I just did a quick search and didn't find anything definitive to answer your question but here's my speculation based on this post by Hive account@r-a-h-u-l

https://steemit.com/payoutdecline/@r-a-h-u-l/payout-decline-on-steemit

I would agree that the reason you sometimes see a line through the reward amount is because the author chose to decline payout for whatever reason. The post above speculates that while this may seem crazy to decline payout, maybe the post content is promoting something non-steemit related. The "decline payout" is offered as one of the three options when you're posting (one is 100% power up, one is 50/50 SBD / SP payout, and the third is decline payout). I usually do the 100% power up option but that's purely based on my own strategy here.

I had also considered that maybe it's a post that's been flagged or something but this doesn't seem to be the case.

You bring up a good question as I have seen some of these posts with a line through the payout as well and wondered why that is.