Youtube wanted to end the trend of "unsubscription parties" with streams of a Youtube channel's subs declining in real time. To do that they made changes to the API to only show subs in increments of a 10, 100, 1000, etc Which is why scrapers like social blade can only show perfect changes of 1000 and Youtube channel subs and rounded displayed are rounded to similar standards.
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You'll notice you can't see how many subs he has down to the hundreds and tens places and hovering over it doesn't reveal the exact numbers like it used to. Let alone is it openly displayed like it was before that change. So yeah, it is to do with Socialblade and Youtube themselves, not that mass unsubscription just so happens to be particularly orderly these days.