In this case, it's because the former is explicitly borrowing the latter!

There's been some debate about when exactly the days of the week borrowing took place - it's most likely 2nd-4th century, and therefore a broadly pre-Christian, Proto-Germanic thing, but as late as the 7th-9th century has been proposed (requiring independent borrowings in Scandinavia and England). What's generally accepted, though is that Monday, Tuesday, etc. are calques (translated loanwords) of Dies Lunae, Dies Martii, etc.

Obviously, whether this makes them useful to identify broad domains of the gods in Germanic religion is a bit tricky - it depends on when exactly the borrowing occurred and the extent to which it was led by Roman perspectives vs Germanic ones. Neither of those are clear. But, I would suggest relatively confidently that it suggests a desire to be more Latinate at the time of borrowing, as Germanic societies modelled the way they tracked time on Rome.