I imagine it is because they are not correct enough... the top 50 are there because for the most part they are correct and in the right order, ioi can track the better selling games more accurately and so the top 50 that you see is likely close to the real top 50.
Any games that sell below that may be accurate enough in themselves (which is why they are included in the WW totals) but there may be other European games that ioi has not included or can't yet track, which would take up those positions.
I think I may have just made a total mess of explaining it..
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put it like this:
VGC top 50: accurate enough, very few currently untracked games would make it into the top 50.
Real top 50: might show a few games that ioi doesn't have data for, but will generally look similar to VGC.
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VGC below 50: each individual game will be ok, but some games will be missing.
Real below 50: the missing games would in reality be scattered all over the charts below the top 50, so games that ioi might have pegged at 100th in the chart, might in fact be 130th or lower.
