Why not 1138x640? It would be true 16:9 aspect ratio?! Ridiculous rumor...
Neither 1138x640 nor 1136x640 are "true 16:9 aspect ratio", but 1136x640 is actually more appropriate for scaling.
16/9 = 1.77777778
1138/640 = 1.778125
1136/640 = 1.775
They chose 1136 because it's divisible by an integer (71) that's also a multiple of 639 (very close to the horizontal resolution).
16*71= 1136
9*71 = 639
If they chose 1138 they would have to scale using a floating-point factor (71.125):
16*71.125 = 1138
9*71.125 = 640.125
And the horizontal resolution scaling result wouldn't even have been an integer (640.125), so it wouldn't have worked well.
Basically, 1136 is the best choice but that would mean they would have to leave one line of pixels black while playing a 16:9 video. People wouldn't notice, just like people don't notice the rounded corners (in most default apps, but especially noticeable in the keyboard) are actually just black pixels being displayed.
It's very hard to determine whether the black is from the actual screen or from the tiny black border around it under the glass. Take a look at those black keyboard corners on your iPhone screen and you will see what I mean. You would swear they are part of the black border, but they are actually displayed on the screen with black pixels.
tl;dr: 1136x640 is more appropriate than 1138x640 for scaling 16:9 content. Neither is perfect but 1136 is easy to scale as long as you sacrifice 1 line of pixels while 1138x640 is hard to scale no matter what.
. It will complicate developer's life
