DCTDecode filter used for image with Indexed color space?

I've seen examples of images in PDF that have BitsPerComponent=8, with an Indexed Color Space, but the filter is DCTDecode. This doesn't seem intuitive to me, as JPG supports 24-bit RGB and 8-bit gray. I haven't heard of 8-bit color JPG so I'm just wondering how this is done?
I'm just trying to understand how this works if someone could please indulge me with a technical explanation it would be most appreciated. I understand how the image is represented using the colorspace, I just don't understand how this filter is applicable to 8-bit data.
Please note, I understand that it's not an actual JPG file, that the data is only using the compression scheme.

DCTDecode is entirely legal for 1-component (greyscale) samples as you've noted. There is no problem there.
Remember, that applying a filter (and later decoding it) is entirely separate from the meaning of the data. So it's possible to use filters that just don't map correctly to the data. For example, greyscale DCTDecode could be used to encode samples which were, on decoding, to be treated as RGB. It wouldn't compress well, but the meaning isn't affected.
But DCTDecode for an Indexed colour space is very unusual. Don't look for special treatment, it would be enconding the image data, which would be offsets into the colour table. And of course DCTDecode is lossy, so decompressed data should be close, but is not always equal, to the original.
In most cases of Indexed colour, this would lead to random and useless changes to colours. But it is entirely possible to have colours which map into a linear and smooth set of shades of the same colour - such as 256 shades of pink. An image using such an Indexed colour space is a candidate for using DCTDecode.
In using filters, it is best to forget what you think you know about the data, and just follow the precise rules of the filter.

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Thanks Geoff, that was very helpful!
Date: Sun, 13 Oct 2013 19:20:07 -0700
From: [email protected]
To: [email protected]
Subject: Searching for images with no keywords in large catalogs - LR5
    Re: Searching for images with no keywords in large catalogs - LR5
    created by Geoff the kiwi in Lightroom for Beginners - View the full discussion
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