Why does the Batwoman show uses this aspect ratio?

A question I forgot to ask:
Why does the Batwoman show uses this aspect ratio?

From the first trailer to the actual episodes know I noticed always black bars on top and on bottom on my 16:9 screen.
Shows like Arrow and Flash from the CW doesn't have that, the whole screen is filled.
I measured the picture size (without the bars) on my screen and came up with 18:9, the format that new smartphones seem to use...
So why does the Batwoman uses this format? And please don't tell me that the show is optimised for smartphones!
-
*EDIT*
Well, I learned that a lot of new TV shows use this format now, so it isn't as unusual anymore....