It shouldn’t be a source of huge discrepancy. Some, but certainly not the gap that Aiko is seeing.
If a ganker dies to CONCORD and doesn’t post the loss, then it won’t appear on zkillboard, but also doesn’t appear in the MER data either.
The MER doesn’t track NPC-only kills. It only tracks player-involved kills.
For example, looking at Catalyst losses in Uedama last month, the MER reports only 10 Catalysts in player-involved losses:
Yet look any day at zkillboard and on most days there’ll be more Catalyst losses then that, that are NPC-only:
However, you can also see that each of those 10 player-involved Catalyst losses in Uedama last month are on zkillboard:
10th:
9th:
8th:
7th, 6th and 5th:
etc.
In most cases, anti-gankers that whore on CONCORD kills, then post the kills (or are linked) to zkillboard.
Additionally, zkillboard stats also only include pvp related numbers and not NPC-only kills.
So the alignment between zkillboard and the MER shouldn’t be that far out for pvp related killmails and neither stats are affected by NPC-only losses.
zkillboard only has killmails generated by the killmail server, as it’s source of data.
The MER uses the same source of data, but doesn’t miss any. Every player-involved killmail ends up in the MER, but not all of them end up on zkillboard.
The difference between zkillboard and the MER for many ships is small. Here is a comparison over the first 6 months of last year, but the figures hold when you go back and/or forward further also:
However, for some ships, the data on zkillboard isn’t anywhere near as good.
Overall, we know that zkillboard receives between 40-50% of all killmails generated in the game, but most of the missing killmails are losses purely to NPCs and often new players that aren’t linked to zkillboard - hence why the real griefers causing new players to quit, are NPCs. They indiscriminately kill with no benefit to themselves and don’t even take the time to teach the new players what they did wrong or how to get better. NPCs are true scumbags respponsible for more loss of new players than any pvp related activity is.