I had the same problem.....there are plenty of times when you bring in an image from a camera that defaults to 72dpi at 30" x 40", depending on your sensor size. Normally the first thing I do is change it to 300 dpi without any interpolation, i.e. unchecking the resample box. I believe where the confusion is coming in is that Affinity is able to do it, but it does not show you in real time, like photoshop does. When you're in affinity photo and you go to resize image and uncheck the box, just change the resolution to 300 (30 x 40@300 dpi) and hit resize and it will change the size of the image to 7.2" x 9.6" @ 300 dpi without any interpolation. The problem in Affinity Photo, is that it does not show the change until you hit resize, contrary to photoshop, that shows in real time how you're resizing the image and affecting the size and/or resolution. Will this be fixed in future updates? I'm hoping it will be. I hope this sheds a little light on the confusion.