Understanding which wireless network is actually the best

Every year, a bunch of different studies come out that crown a winner, the "best network" in the USA. Each study claims to use the best, most scientific methodology to give "unrivaled accuracy" or "undisputed results," or something else equally quotable.
But if the studies are so good, why do they give such different results? Take a study published today by RootMetrics, which has Verizon in first place and T-Mobile in last. That's in stark contrast to a study last month from OpenSignal, which ha