I can't say I noticed the black/white on that photo before, but I see your point. It's just the way the splices have been made though.
Twisted pairs are used for a lot of replacement overhead drops these days (at least in my area -- Norfolk), but there are still many older service drops like the opne above in use. The house that transformer feeds is an old one, not newly built, so it's likely the transformer is just a fairly recent replacement which was linked to the existing single service cables.
