Draw is great for creating web graphics but there are some stuff you must do for the Web Image Optimizer to behave like expected. You need to:
The key to this is how you have calibrated the rulers (it resides under the zoom tool settings). It does not necessarily have to be the same as the real screen resolution, but if you want WYSIWYG it's necessary to set ruler calibration figure to the same as the actual screen resolution. The page resolution and the ruler resolution are corresponding so changing one changes the other too. But that figure does not really have any impact om the WIO output. It only comes into play when you use the export route, in the Convert to Bitmap dialog. (Like if you have the page resolution set to 300 ppi, a one-inch square will be 300 pixels wide in the Convert to Bitmap dialog @ 100%).
Example: My monitor has a dot pitch of .25 mm so it is very close to 100 ppi. I therefore calibrate the rulers deliberately to 100 ppi. Now when I draw a square of 1x1 inch its really 2,5 cm or 25 mm on the screen when zooming 1:1. It will also be 100 px wide in the WIO because I have told it so when I calibrated the rulers. The WIO calculates from the ruler calibration figure. So If you instead set the figure 72 ppi in the ruler calibration, the 1-inch square measures 72 px in the WIO when zooms 1:1. But then it will only measure 18 mm instead of 25 (72x.25=18). Of course you always zoom to 1:1 when outputting through the WIO because otherwise you will never know what you get. I'd say the WIO is designed for web work and then you will want to design in WYSIWYG mode so everything has its real-world measurements.
Some monitors, like the popular 19 inch ones at 1280x1024 pixels, have a larger dot pitch @ .27 mm and so the ruler calibration figure should be set to 94 ppi to get exaxt WYSIWYG.