The Gemora rules: It is obvious that if he designated all of his property to his adult son, we assume that he only appointed him to be a guardian (for he would not want to take away his other children’s inheritance).
The Rishonim ask: How can our Gemora say that the father’s stipulation is not effective when the Gemora above (130a) explicitly rules that the halachah follows Rabbi Yochanan Ben Berokah?
The Rash”bam (ibid) says that the halachah follows Rabbi Yochanan Ben Berokah only when the father uses the expression “as an inheritance”; however, our Gemora is referring to a case where he used “as a gift” expression, and in such a case, the Gemora here rules that his real intention is to make him a guardian.
The Ram”ban answers that the halachah follows Rabbi Yochanan Ben Berokah only when the father stated his stipulation, however, our Gemora is referring to a case where he wrote it down.
The Ro”sh quotes Rabbeinu Chananel who says that before, the Gemora was referring to a case where he wanted to increase the portion to one son and decrease the portion of another; he did not give his entire estate to one person. Here, we are dealing with a case where he gave one of them his entire estate. In such a case, we may assume that he meant for that person to be a guardian on the estate on behalf of the others.
The Rash”ba explains that the Gemora before was discussing a specific case where he clearly said that he wants that particular person to inherit him and he does not want him to be a guardian.
The Baal Hameor understands our Gemora to be referring to a case where the sons are minors, or they are overseas. In such a case, we assume that the designated son will be appointed as a guardian.