<…> if my memory is not wrong, that time coincided at the end of 2017 the merit system came into effect.
January the 25th 2018 really.
A uniform amount of merit as you mentioned above (100, 250, 1000, 1250) is given by forum admin based on their activity count and rank before the merit system was implemented.
10, 100, 250, 500, 1000. It was really based on rank solely. Some Hero Members did receive 1K Merits instead of the corresponding 500 Merits.
The initial amount of sMerits airdropped, on the other hand, did depend on Activity as an additional parameter to Rank at the time.
<…> after getting a uniform amount of merit they leave this forum.
I doubt Merit had any influence on grandfathered account that kept their Ranks and received an amount of Merits in accordance to their rank. The airdrop applied to all ranked accounts at the time, regardless of whether they were active or not at the time (coherence between Rank and Merits). Those leaving or having left would do so/have done so anyhow.