In Azure, why is reserved basic public IP cheaper than dynamic basic public IP?

From here:

https://azure.microsoft.com/en-us/pricing/details/ip-addresses/

Not quite sure why the reserved IP costs less? I assume this is by design, but if a student asked me why this is the case, I'd have no explanation. Anyone have any ideas or a link to documentation? Thanks.

Per the documentation,

• Static/reserved IP is assigned an IP address at the time it's created. The IP address is released when the resource is deleted.
• Dynamic IP isn't assigned to the resource at the time of creation but is assigned when you associate the public IP address with a resource and is released when you stop, or delete the resource.

Looking at the above, the static IP can stay unused which means it may be available but may not be used in a running deployment. However, dynamic IP is always assigned to a running deployment and is never left unused as it will be released as soon as the IP is disassociated or stopped.

Hence, Azure charges a nominal fee for unused reserved/static IP addresses.

Refer the FAQs under the pricing table: https://azure.microsoft.com/en-us/pricing/details/ip-addresses/#faq