Activity is, in its simplified form (there are some additional complexities that need not be brought up now), derived from the number of posts you make within each 2 week window, with a max. of 14 per window.

A person with 1K activity could have generated all/part/some of that Activity before the Merit System kicked-off, and therefore a bunch of his posts would typically not have postulated to being merited. More typically though, you’ll find that such 1K Activity 0 Merit type accounts are busy bounty hunting, and the derived posts are therefore not merit worthy.

I doubt you'd find many post Merit System kick-off accounts, with 1K Activity and 0 Merits that do not fall into the bounty hunter spectrum.