It might have previously used a different card. It wouldn't be nice if the numbers were determined by the random order in which the devices are detected, so the software remembers which MAC address belongs to wlan0 or wlan1. Other udev configurations make it name the devices after the physical PCI ports in which they are inserted.
You might change this by editing or creating /etc/udev/rules.d/70-persistent-net.rules.
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