Downstream Correctables and Uncorrectables are caused by noise on the downstream channel. Docsis contains an error correction mechanism (FEC) to correct octets when corruption is not too great. Having some Uncorrectables/Correctables are OK but you want a very low percentage.
See my post in the link below for some links to articles that give a more in depth discussion of Correctables and Uncorrectables.
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Re: How do my Signals Look?