Question: Normally, reputation increases by 10 if an answer gets 1 upvote, 15 if an answer is accepted, and 5 if a question gets 1 upvote. 

But on this website, reputation increases by 1 for some of my questions. I'd like to know how reputation increases by 1. Thanks.

Answer: 
Your reputation may increase by 1 if an answer you downvoted is deleted.

You can see all the individual events that affected your reputation by going to your Profile->Activity->Reputation.
It's also possible that you've reached the reputation maximum for the day.

In general you can only gain 200 reputation per day (although some things bypass this: bounty awards, accepted answers, and association bonuses). For instance, if you were already at 199 (somehow), then getting an upvote would only give you one more point (+1), not ten (+10), even though it *would* have given you the additional nine points if you hadn't reached the daily maximum.