See my answer below, but this answer is scapegoating. If it were printer driver issues, it would fail with all Apps, Apple and third-party. This happens in Apple apps, not in third-party apps. Apple should not be imposing manual margins, if a specified page size and margin from the printer driver is present. If it were an "any printer" driver that lacks specified sizes and margins, it is a different story. But, the fact that the same result happens in saving to PDF proves it is in Apple's process of rendering the page out to the driver. What Apple is effectively doing is creating (imposing) a false frame to the page, attempting to keep elements from interacting or violating a margin. This makes sense for a device or output profile that needs strict margins, but should never be imposed on devices or profiles where those settings are present. This test can be proven by setting the margins to 0 and printing. The result should be that printing goes to the physical limitations of the printer. Instead, it cuts it at Apple's desired approximate .875", .75", .75", .625" margins/gutters.
My compliments on the workaround, but setting a custom size has mixed results and disables the ability for duplex printing for many printers, unnecessarily wasting lots of paper. It is sad that even Microsoft does a better job at this than the co-founders of Postscript universal drivers. Apple is getting lazy and when called out, find the easiest way to blame something else, rather than take responsibility and fix it.
