That's a good question. Without digging into the specifics of their reasoning, I would say it is unusual to have Nino-3.4 sea surface temperatures so warm (latest weekly index values around 1.2C) without a clear El Nino response in the atmosphere. Back in May, I speculated that this event may punch below its weight during its developing stages because the global oceans outside central-eastern tropical Pacific are so warm. Perhaps that is what is occurring now? I will also note that there have been some El Nino-like tropical atmosphere conditions, so I wouldn't say there's a total absence of evidence of ocean-atmosphere coupling, but the evidence is not as strong as we might expect for such a warm central-eastern tropical Pacific.