People forget to clean them up after shutting down the instance (probably kept in case you want to keep your IP)
Considering the whole IPv4 extinction that's coming down and all.
Not saying it can't happen, but I've never seen an 'Other' listed on the bill details screen. As I said above, it's detailed to a fault.
I haven't used Lightsail, so I'm not sure if those snapshots end up in the general ec2 console. But, if you already checked the LS console and didn't see anything, then they must be in the ec2 console.
I've seen people mail in checks worth less than the stamp it took to send them. It's a wild world out there.
We do try to determine where the discrepancy started and watch for indicators of a bigger issue.
I have to imagine that these things are manual and a human has to do it, so they'll sort it by the size of the account and do what matters