If I examine the average treatment effect on the treated using a matching estimator, only the treated variables should be matched.
However, when I examine the matching variable (generated by "gen(match) "), I find that all observations (treated or not treated) are matched.
I am confused about why this is the case. Example below.

use http://www.stata-press.com/data/r15/cattaneo3.dta
teffects nnmatch (bweight fage) (mbsmoke), nn(1) gen(match) atet
tab mbsmoke if match1 ~= .