The display resolution that is configured may not be the correct one for
your screen. To solve this:
When multiple displays are connected
Michael Hill
Describe Displays:Presentation Mode when it can be tested.
If you have two displays connected to the computer (for example, a normal
monitor and a projector), the displays might have different optimal, or
native, resolutions.
Using Mirror mode, you can
display the same thing on two screens. Both screens use the same resolution,
which may not match the native resolution of either screen, so the sharpness
of the image may suffer on both screens.
Using Join Displays mode,
the resolution of each screen can be set independently, so they can both be
set to their native resolution.