It's quite simple, some hero member got extra merit when the system was introduced, you can see in here theymos mentioning it:
While specific formula is not given, theymos wrote that the initial sMerit is not solely determined by rank, but determined by rank and activity points obtained in the last year.
The exact formula was:
yearActivity = activity in the last ~year, max 378
freeMerit = the merit (not sMerit) you started with
modifier =
0.1 if member
0.2 if full member
0.25 if sr member
0.35 if hero
0.4 if legendary
return (yearActivity/378) * modifier * freeMerit
So the most you could get is if you were a Legendary who maxed out your activity points in the last year, in which case you would get 1 * 0.4 * 1000 = 400 sMerit.
Note that I later made an adjustment giving 500 extra merit to some hero members, but no extra sMerit was given here.I'll probably publish a dump of the initial sMerit at some point.