The difference is you don't register a domain under github.io, you merely loan it. Some countries, like Poland, have a bunch that are real domain suffixes

https://www.dns.pl/en/list_of_functional_domain_names

▲

degamad 3 days ago | parent [-]

Loaning or renting (registering) amount to the same thing for the purposes of the the public suffix list: because the *public* can create entries under github.io, you cannot assume that alice.github.io and eve.github.io are controlled by the same entity, so you should not share alice.github.io's data (e.g. cookies) with eve.github.io.

	▲	whalesalad 2 days ago \| parent [-]
		There is no formal ICANN TLD list. The PSL is your best shot. So it is actually wrong to assume that your situation is the sole purpose. For instance, https://data.iana.org/TLD/tlds-alpha-by-domain.txt Where is .co.uk ? That is - for all intents and purposes - considered a TLD. So PSL is currently doing double-duty and the distinction is very important.