Addresses are handed out in blocks. A block written 203.0.113.0/24 holds 256 addresses that share their first three numbers. Because one owner usually controls a whole block, a site that sees abuse from several addresses in it may act against all of them.
This is the weakness of buying a list of datacenter addresses from one block: they can all be blocked together. A rotating pool spreads requests across many blocks and many networks, which is one reason residential traffic is harder to filter.
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