Photoshop has no margin settings because it is an image editing app, not a page layout app. All it does is read the information from your default printer driver.
The margins you are seeing are "hard coded" by your printer driver since those are the limits at which it can print. Depending on your printer, you *may* be able to print "edge to edge", but not all printers support those features.
The printer must push paper underneath the print heads. At some point it runs out of paper to push, resulting in a higher "back end" margin (in your case, .46 inches) than on the sides or the top.
You can fool the printer by defining a custom page size but that is not recommended (that's why they hard coded the margins in the first place).
Most folks define an image smaller than their paper and trim.