Apple's bombshell warning about a virus's impact on its business falls into a unique category for me: I'm shocked, but not surprised.

COVID-19, the novel coronavirus, had already emerged as a serious concern in the days before Apple's last earnings call three weeks ago. CNBC reporter Josh Lipton and I always talk before the quarter to flesh out our thoughts about the most important metrics to watch and forces to consider.

I told him I was concerned about the virus' effect on Apple for two reasons: The virus epicenter in China is a nexus in Apple's supply chain, and delays in the movement of components could hamper Apple's ability to assemble iPhones. Also, if Apple had to close stores, it could stifle demand for iPhones, and it's not clear to me what happens to demand for a luxury good like that once it's stifled. If you're in an environment where you're on furlough from work, your paycheck is uncertain, and you have new virus-related costs, are you still going to buy an iPhone? Maybe not until the situation has stabilized for a few months.

The key question was how Apple would forecast its results based on a volatile situation with an unknowable outcome. Apple could have chosen not to give guidance at all. It could have guided as though the virus would have a muted effect. It could have taken an approach that factored in the possibility of a multibillion-dollar impact.

It chose the third option — estimating the impact. Apple reported, and Wall Street cheered its forecast. Guidance for the current quarter was wider than usual because of the uncertainty attached to virus fears, but still strong. The stock matched the excitement.