How can an iPad in airplane mode detect iOS update?

I have an iPad Air 2. It only has Wi-Fi capability and I keep it in airplane mode as I don't want it linking to my iPhone or other Apple accounts and disrupting any in progress memo recordings. Well, the iPad has been in airplane mode for the last 1 week. This morning I saw on my iPhone that an update was available so I went ahead with that update. When I accessed my iPad to update and went into settings to turn off airplane mode I noticed that it had already detected that an update was available! How is that even possible when it had been in airplane mode all week?