Statistical factors: the sample size is so small that it could be less effective and still show up this much more effective.
I hope not, but sadly it is a real possibility that needs further study.
This vaccine is an inactive host virus whose payload has been replaced by the coronavirus "spike". It has always been a worry that the body will generate immunity to the host virus and thus make the vaccine less effective. That is a likely explanation for the first-full-dose regimen being less effective.
Shouldn't it warrant a research if even smaller dose has even higher efficiency?
Yes. But clinical trials are expensive ($100m plus) and time consuming.
If both regimens underwent Phase 2 testing, there should be data on comparative immunogenicity.