Macbook identifies 21" monitor as 13.9"

Macbook Pro 13 inch retina 2015 can't properly identify an external VGA monitor (Viewsonic VA2013w). The mac thinks the display is 13.9 inches (is actually 21") and therefore offers the wrong resolutions. It is connected via a VGA to Mini Display Port adapter and functions perfectly with other macs Ive tried it with. The native resolution is 1600 x 900 but 800 x 600 is offered and 1600 x 900 is not an option been if i option click scaled.