Oricon manga reports tend to fall well, well short of the “actual” values reported by publishers. I’ve written before about one of the most extreme cases (Yuruyuri’s one million copies with 0 weeks in the charts), but there are plenty of others out there. This isn’t a comprehensive look at it (and these series may or may not represent “average” cases of underreporting); it’s just me picking a couple confirmations and counting up the total (i.e. total on weekly charts from last week a volume appeared) Oricon reported sales up to the point when the publisher reported a given number.
Some of these may simply be unsold copies, another part may represent copies sold at a 10k/week rate in the long-tail shadow of the charts, and a few may come from the overseas sales Oricon chooses not to count. How to separate those three is anybody’s guess. I’m not trying to do that, just give some idea of how big these gaps can be.
Tonari no Seki-kun
Via Publisher: 3,000,000 (July 7, 2014)
Via Oricon
v1: Off charts
v2: Off charts
v3: 107,945
v4: 160,438
v5: 171,822
v6: releases July 23, 2014
Via Oricon Total: 440,205
One Punch Man
Via Publisher: 3,200,000 (April 20, 2014)
Via Oricon
v1: 451,352
v2: 416,564
v3: 383,183
v4: 360,277
v5: 335,625 (503,098 on half-yearly ranking)
v6: Released after publsiher claim
Via Oricon Total:1,947,001 (2,114,474)
[Note: A half-yearly figure recently pegged OPM’s as selling 1.6 million copies in that period. However, this also includes the release of volume 6, and doesn’t cover the entirety of the series’ run.]
Psycho-Pass – Kanshikan Tsunemori Akane
Via Publisher: 380,000 (November 3, 2013)
Via Oricon
v1: 76,109
v2: 56,500
v3: 70,107
Via Oricon Total: 202,716
[Note: v3 is included, in spite of being sold after the claim was made, because the 380,000 figure was included in volume 3 and is presumed to include v3’s first pressing.]
WataMote
Via Publisher: 1,500,000 (July 19, 2013)
Via Oricon
v1: Off charts
v2: 91,466
v3: 172,027
v4: 140,743
v5: Released after publisher claim
Via Oricon Total: 404,236
Animetics 
Denpa Kyoushi (25%)
Via Publisher: 1,000,000 (Dec 4, 2013)
http://kissmanga.com/Manga/Denpa-Kyoushi/100-A-battle-among-the-Leader-Pairs?id=181468
Via Oricon
v1: Off chart
v2: Off chart
v3: 30,980
v4: 21,872
v5: 42,520
v6: 35,849
v7: 38,312
v8: 30,089
v9: 23,191 + 21,127 (sp)
V10-12: Released after publisher claim
Via Oricon: 243,940
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Looks like under counting is particularly severe for Oricon manga compared to Oricon anime. More problematic than simple undercounting, though, the deviation from publisher percentages vary wildly from one title to another. Because of those two issues, simple sums of Oricon weekly manga charts really looks like a bad dataset to work with.
At least anime sales deviation tend to be more consistent.
At least manga sales are consistent between volumes of a given series. I would write about manga a lot more if more and more accurate sales data were available – the various weekly shonen magazines are really far down my list of manga topics I enjoy, and there’s rarely good data for much else.
The worst part of all of this is that the undercounting is likely due to a combination of factors in some weird proportion. There’s the sub-10k long tail abyss. Oricon doesn’t count overseas sales (could be big, especially since most of Asia is an overseas market). And we don’t know if localized publications are included in these publisher claims or if they’re just domestic (Taiwan picked up the WataMote manga within like 18 months, so theoretically those sorts of things could be included). I feel like it’d be possible to deal with some of these issues if there was just one, but they’re all potentially significant. There would be ways to look at each of those individually (amazon books rankings, evidence of overseas interest in a series that might lead to exporting, etc.), but together they’re just wicked hard to unravel. I mean, I choose to look on the bright side because all this means that manga are doing better than they seem to be, but it is pretty bleh to see a discrepancy like Seki-kun/OPM, where series of near-equivalent print runs appear to differ by a factor of 4 in volume.
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