70% is average from two distinct approaches here: one is 90%+ (which is great), and another is 62% (which is top end of flu vaccine).
Maybe it makes medical sense for half doze/full doze work better than two full dozes, but it seems that organisers of the trial were surprised also.
SARS2 is much more infectious than flu, we need far more than 62% efficacy, especially given growing number of Mor Ons who won't take any vaccine.