Quikrete vs. sakrete
Quikrete and Sakrete are essentially the same products made to minimum mortar or concrete standards and there are not "secrets". They both vary somewhat because they are made from the locally available aggregates that do not have very specific ASTM standards (ASTM C33). Even the preferred proportioning specifications have wide overlapping standards and the concrete strength standards are for lab tests and not actual in-place strengths.
Concrete cures/increases in strength as long as there is moisture available. If a mix says it includes Type III (High Early) cement, it will have higher early early strength, but possibly a lower ultimate strength, but strength is really not important for a fence post foundation.
I had a friend that packaged both Quikrete and Sakrete in the same plant with the same aggregate and cement. He also packaged his own brand plus bagged kitty litter in slower periods.
For a fence post it does not make much difference, but make you feel better.