The main difference is in the objective of the study.
The All branches research includes the direct ancestors of the primary person in each of the ascending lines, without distinction of sex or surname.
That means, father and mother, four grandparents, eight great-grandparents, sixteen great-great-grandparents, etc. therefore doubling the number of ancestors from one generation to the previous one.
Surname research is limited in each generation to the direct ancestor with that surname and their wife.
That means, father and mother, paternal grandparents, paternal-paternal great-grandparents, etc. so that in each generation there will be only the two people who pass on the lineage.