One page as a data point is way too small to conclude anything. It could totally be some uploaders just happened to dump a batch of illustrations with males since he/she likes it.

There are tags for male/female which will be a better representation.

    1girl: 3038k
    1boy: 559k

(Note: these tags are used when there is only 1 character with this gender presents. But it's good enough.)

Your idea about they filtered images could still be true, to be clear.