What affects LRFD strength reduction factors?

In general speaking, the strength reduction factor reflect the confidence on a structural element's ability to withstand a certain type of loading. It is derived statistically from both the research experiments and the past experiences; and it is based on the structural response/behavior of a certain type of material subjected to a certain type of load. In this sense, you are correct in calling it a "resistance factor", which is always less than 1.