Understanding TTL values in DNS query answers using dig
Solution 1:
There are multiple boxes behind 1.1.1.1 or any other similar public DNS resolver. Hence, technically, all those boxes can share their DNS cache, or not.
If they don't, each one will have different set of records cached, and different TTLs associated to them.
What you observe may seem puzzling but has 0 negative effects on the DNS service.