It takes some time between unique (or in some similarity cluster or whatever they have) content is uploaded, reported and taken down. During this time, exactly same content is not being taken down, because it's not known to be bad. So even if all the content was byte to byte identical, if 20% of it is uploaded before it is reported, there is no chance to have >95% block rate automatically.
Once content is known to be bad, people (and bad actors) will usually stop uploading it (because it no longer works). Also people stop sharing it, because it's no longer possible (and FB delete all instances of such video from the platform anyways). If there is lame script kiddie that does not handle errors, then the number could be 95%, because it would continue with upload attempts. But normal people, or sophisticated actors will respond to errors and stop uploading exactly that video.
1 extreme example: with 1 sophisticated bad actor uploading exactly same video (through some fake/hacked accounts/pages, or whatever), the block rate will always look bad for Facebook: bad actor uploads N videos before being blocked. Once blocked (on video N+1), bad actor will stop. The block rate will be here 1/(N+1), which will be way less 80% (FB's current case) for any N > 0.
What did I try to say? Percentage of what is being blocked does not make sense, since it also depend on how fast is attacker detecting that is being blocked. If fast, the number will always look bad. If attacker does not give a shit, the number will look awesome.