Something else to factor in is the TTL of both NS/A types for each apex domain and the individual records including sub-domains. Clients will not be querying Quad9 until the TTL expires on their clients. TTL would have to be factored into query rates to determine popularity correctly whereas these lists just show raw query numbers.

For example, there are many records under amazonaws.com that have 5 second TTL's mostly EC2 instances. As such clients will query them at a much higher rate whereas grammarly.io have a number of records with a 900 second TTL. This will skew the ranking positions of the two apex domains. I suppose if one wanted to game this they could have an A record to a non-critical part of a site that is not visibly rendered by the end-user and has a TTL of 1 second assuming quad9 is not rewrite min/max-ttl which some resolvers do.

Examples of just some of the TTL's used on these apex domains excluding individual records:

    30 32 60 300 600 900 1200 1800 3600 7200 10800 21600 28800 43200 86400 90000 3600000

Some examples of rewriting max-ttl I forgot which ones rewrite min-ttl:

    for Resolver in 1.1.1.1 8.8.8.8 9.9.9.9 216.128.176.142;do echo -en "${Resolver}:\t"; dig @${Resolver} +nocookie +noall +answer -t a big.ohcdn.net;done | column -t
    1.1.1.1:          big.ohcdn.net.  3628800  IN  A  227.227.227.227
    8.8.8.8:          big.ohcdn.net.  21422    IN  A  227.227.227.227
    9.9.9.9:          big.ohcdn.net.  43200    IN  A  227.227.227.227
    216.128.176.142:  big.ohcdn.net.  3628800  IN  A  227.227.227.227  # authoritative server

[Edit] I just realized they made a general statement to this effect in the git repo.