I'm having the same issue.
I have been using the same monitor with my HP Envy 15 3090ca before I updated and it shows the resolution 1920 x 1020 when I connect and extend the screen.
After the update, my laptop screen sizes say they are 1920 x 1020 however when I put the start screen up on the external monitor or any app, it changes the resolution to something smaller. The desktop looks fine, until I drag my browser window from my laptop
screen to the external and all of a sudden the content in the browser zooms in huge!
I adjust it then bring it back to the laptop screen and it shifts again but because I zoomed out in the previous screen, the browser content on the laptop compensate and is zoomed out also.
I like for it to be back to normal.
I am running a AMD Radeon 7xxx model too with switchable graphics; hopefully if it is AMD, they produce a updated driver for Windows 8.1 or Microsoft has another trick up their sleeves.