As someone who lives in a place where I have to check any black spiders I see to make sure they’re not black widows, that’s the first thing I noticed, too. It’s not like drawing the hourglass on the bottom makes it harder to see/recognize, especially when the text explicitly identifies it as a black widow.
I guess the artist has never seen an actual black widow and this is done out of ignorance rather than as a conscious design choice. It’s possible that there are multiple subspecies of black widow, some of which have the markings on the top, but I’ve never heard of that. Then again, I’m no biologist…