By default Airplane mode disables all wireless frequencies on your device so that you can comply with airline regulations. This means that any radio frequencies relating to cellular, Wi-Fi and Bluetooth are disabled.
However, when your device is in Airplane mode, it is actually possible to manually enable Wi-Fi and/or Bluetooth. And, if you do, the next time you're in Airplane mode these settings remain.
So, the most likely answer is that at some point you activated Wi-Fi during Airplane mode (either intentionally or by accident) and that ever since your iPad has sill been connected via Wi-Fi whenever you've activated Airplane mode.
As per Apple:
Use Wi-Fi and Bluetooth in Airplane Mode
If the airline allows it, you can use Wi-Fi and Bluetooth while in
Airplane mode. You just need to turn them on separately. On your
iPhone, iPad, or iPod touch, you can turn on Wi-Fi or Bluetooth with
Control Center. Open Control Center from the Home screen and tap [the Wi-Fi or Bluetooth icon]. You can also go to Settings > Wi-Fi or Settings > Bluetooth. To use
Wi-Fi and Bluetooth on your Apple Watch, just turn off Airplane Mode.
If you turn on Wi-Fi or Bluetooth while you're in Airplane mode, they
will be on the next time you use Airplane mode, unless you turn them
off while in Airplane mode.
Source: Use Airplane Mode on your iPhone, iPad, iPod touch, and Apple Watch, Apple. [Bold emphasis mine, words in italics added for clarification]
The following screenshot is from an iPad in Airplane Mode and connected to Wi-Fi at the same time: