Yes, good question. This is a common confusion people make when trying to understand rational ethics of confusing intrinsic values with objective values. If the good is inherent in some aspect of reality, then you’re right that it follows every person employing the same method must come to the same moral conclusions across the board.
But this isn’t precisely what Objectivism holds. It is true that the nature of entities have objective (mind-independent) properties that have certain specific and necessary effects, and that this is important for Objectivist ethics. But Rand doesn’t hold morality inherent in external reality alone, rather it is relational. This is the important aspect of agent-relativity in Rand’s ethics, which takes account of the beneficiary of the action: rightness or wrongness is an aspect of reality in relation to man. In that way, what is good for man is contextual (can differ from person to person, situation to situation), objective (absolute and mind-independent), and agent-relative (dependent upon its effects on you). In other words, each of the abstract, substantive goods and virtues will apply differently and contextually to each specific individual person.
So to achieve the good means that there are individualistic potentialities to be developed. This makes human flourishing always unique to a specific human person. Each person’s life to live is his own. To seek a good human life is to seek self fulfillment through the actualization of one’s unique faculties.
For details, see OPAR chapter 7, section 5 “Values As Objective”; Smith Viable Values, chapter 3 “Intrinsic Value: A False Foundation”; Smith Ayn Rand’s Normative Ethics, chapter 2, p. 25.
Rand also notes that, if one did hold that the good was embedded in certain things, then this leads to acceptance for using force to achieve the good:
“The intrinsic theory holds that the good resides in some sort of reality, independent of man’s consciousness. If a man believes that the good is intrinsic in certain actions, he will not hesitate to force others to perform them. If he believes that the human benefit or injury caused by such actions is of no significance, he will regard a sea of blood as of no significance. If he believes that the beneficiaries of such actions are irrelevant (or interchangeable), he will regard wholesale slaughter as his moral duty in the service of a “higher” good. It is the intrinsic theory of values that produces a Robespierre, a Lenin, a Stalin, or a Hitler. It is not an accident that Eichmann was a Kantian (CUI 22.)”
See also these threads: “Is it moral to lie to an enemy?” and “Does different knowledge justify different morality”