Congrats xebec! So many variables determining. Minimally you require adequate evidence of ability to support, accommodation and relationship. Yet, those things in themselves do not determine the time lines or whether your partner is called for an interview. Time of year and ECO profile/circumstances are also key.
I think it has been tempting for some with good outcomes in the past to suggest that they did everything right which is why it happened for them. However, there are many on here who have been the key sources of the info that we all rely on for our application who had terrible experiences getting their wife/fiancee a visa. so there are many factors that as you suggest, we will never know about.
Congrats again!