Consensus is the only one of the four Sunni sources that cannot be checked against a fixed text, and the schools disagree less about its authority than about how anyone could ever confirm it happened. Early jurists faced an obvious problem: the Muslim community was already too large and too scattered by the second Islamic century for any scholar to canvass its opinion directly, so claims of ijma rested on silence as much as on recorded agreement, the assumption that if a ruling had been publicly held and no qualified scholar is known to have objected, agreement could be inferred.
That inference is exactly what Ahmad ibn Hanbal is reported to have distrusted. Sources attribute to him a refusal to accept a claim of consensus reached after the generation of the Prophet's Companions, on the reasoning that later scholars were too numerous and too dispersed for anyone honestly to have surveyed them all, so a claimed ijma after that point is really one jurist's impression dressed as the whole community's voice. The distinction mattered practically: a ruling resting on Companion era consensus was treated as close to certain, while one resting on a later, unverifiable claim carried real jurists' names behind it but not the same evidentiary weight, which is why disputes over dating a given consensus recur throughout the historical formation of the Sunni schools of law.