I can confiorm that the rating a thread gets is the same as the OPs rating at the time of post.
User ratings are more complex here than Ubb. They involve a forum moderation system, user rating, ranking, etc.
To compare to the Ubb user rating system, every user was allowed to rate every other user once. you couldn't change this rating over time, you were just stuck with the single value.
On this system, user ratings are a function of the total rating of your posts. You rate the posts - rather than the user directly, the user inherits this karma from their post average. This enables you to rate a person based on what they say over time, rather than just once for all eternity. You can change your mind, you can rate an individual insult down, and an insightful post up.
Every time you post, you rate yourself as well at your current Karma. This a) establishes the base rating for an unrated post. b) prevents you from rating the thread voluntarily, c) adds to your base of ratings to better moderate the effect of ratings on you.
The goal of the system is small incremental changes, with as many people as possible rating posts honestly, with an established base of ratings by the frequent posters. this means you won't see wild fluctuations, just small changes, and the system is designed that small fluctuations rarely change your posting title (Scholar, Sage, Guru).
For people with low post counts who I bump up to high post counts fluctuations become an issue. nothing I can do about this, you just have to post, or get someone to give you lots of "Decent" ratings of 3.0 to even things out.