IIRC this is an issue present in the original as well. So I doubt the second part of your post serves as a possible explanation for this disparity.
When Arcueid describes the Dead Apostle Rankings she says that by around Rank III they're still basically unbothered by sunlight, but by Rank V they start to get anemic from the Sun. Since the Rankings describe the "Severity/Depth of a Dead Apostle's Curse" its probably less something about their nerves, but rather that with the progression of their Curse, their vulnerability to the Sun increases.
I feel like there are a little too few details here for us to draw such a conclusion. As we know, Arach is responsible for building stuff for the Tohno family. Perhaps it has something to do with what she's been creating. I mean, the land upon which the underground lair was built was something owned by Makihisa. Perhaps Arach is responsible for building it? Maybe she's done something similar around the Tohno mansion.