So it’s not so much what the app is doing - it’s a result of things being browser-based. Don’t quote me, but, without any different scaling set up in the OS, it should be 96 PPI.
The app you’re using might be specifying things in inches, but it really comes down to how it’s exporting out the SVG; if it’s specifying things in pixels (or not specifying any units, just numbers, which are interpreted as pixels), that’s where you’re going to run into trouble.
Long ago, GF created the 12x20 hack (it will actually work with any dimension as long as the aspect ratio is correct), and it should pin the work to the top left of the work area.
This is an awesome trick for having things show up exactly where you want them to.