A madh'hab isn't just some arbitrary collection of rulings that a bunch of people decided to agree on.
What follows is a super simplified explanation of the madhahib; the actual details are far more complex than can be gotten into in this answer, but hopefully this is good enough to at least give a birds-eye view of things.
While all madhahib agree that the Qur'an and the Sunnah are the primary sources for all Islamic jurisprudence, there are different methods of taking those sources and extrapolating rulings. This is especially the case when a ruling is on a topic in which no clear text is present, and a ruling needs to rely on weaker secondary criteria. Not all scholars agreed on which (if any) of these criteria should be applied, or on how important they were in relation to each other.
In general, each madh'hab is defined by its methodology, which is typically the one used (if not devised) by the founder on how to choose between and apply various sources.
Depending on the madh'hab, some sources are more subjective than others and there can be differences of opinion between scholars on how to interpret them. Also, sometimes one scholar would have access to sources and information that another scholar lacks, which could affect their rulings. But in general — given the same sources and applying the same methodology — a follower of a madh'hab would reach a consistent ruling.
See also: What are basic (osol al fiqh) differences & similarities between the 4 (existing) Legal Sunni Schools: Hanafi, Hanbali, Maliki, & Shafi'i? Charts?