Hmmm, I'll have a metaphorical whack at this question, I think its because...
Probably since you're logically thinking about the dream-scape, it'll probably be more detailed as you may be looking around at your LD and thinking how realistic it all looks, thus making it look realistic...
And as to why some induction methods make the dream more realistic is probably because you're actually paying attention to being 'pulled' into the dream-scape(WILDing, and similar methods) so your attention may be drawn to the dream-scape more then it is drawn to the fact that you're in a lucid dream(DILDing).
DILDing is an example because you've been randomly 'thrown in' to the LD rather then created it/been in it from the start...
well I hope my ramblings have helped answer your question in some way.

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