What I think (purely IMO) is that high-rep users (the ones with 10k+ rep) had helped many people before. These people know what kind of answers are considered as "good" and will both get upvotes and help the asker. Because those users post great stuff one-in-a-while, most Askubuntu regulars would think that the user will post great stuff all the time, resulting in upvoting stuff from that user no matter what.
New users doesn't know the rules to this site, so they will have to learn how this site works and it's community. Answers that the new user that is considered "Good" might be a spam for the whole community. Some users will go through a "learning stage" that helps them to learn through the stuff in the community and while they are learning, they might post stuff that are great to themselves but not-so-great to the community, which results in lesser upvotes on the question/answer at the beginning. But as they learn new users can know how to write good post and have many upvotes.
You can look at my rep-count graph if you want. At first it takes a while to learn, but after the rules are learnt upvotes seem like a normal thing.
That's wholly in my own opinion, so correct me if I am wrong or you(the reader) disagree.