The times they're online - a lot of people notice people who are active when they are, so if you're posting mostly when lots of people aren't on - you're going to get noticed by fewer people.
How they use the site - some people use the site socially - they'll chat to people, make friends on here, and make themselves memorable and popular - then their answers/questions will get more attention. Others will just simply answer questions, or at least use it less socially - and thus might fly under the radar a bit.
Good advice is subjective a lot of the time. I've seen users who I think give good advice in situations and had lots of people either downvote it - or turn round and disagree.
But honestly - I don't think it's a big deal. As long as the person they're giving advice to takes notice, then goal achieved as far as I'm concerned.