Because it was still part of the WoD launch raid tier, despite not being opened until after release. The reason people generally consider WoD to have only had 2 raid tiers is because only one tier was actually added as a major content patch. Every other expansion had at least two full raid patches (Black Temple then Sunwell, Ulduar and Icecrown, Firelands and Dragonsoul, Throne of Thunder and Siege of Orgrimmar).
BRF is the major raid in the first of two WoD raid tiers, not its own tier.