Checkboxes and Radio buttons don't show on printout, but do on pdf

As the title says really.

I have a web page with various checkbox and radio button options that help create a quote. This needs to be then printed out.

This all works fine if I print to PDF - the page is shown exactly the same. However, if I print to an actual printer, checkboxes and radio buttons aren't shown.

I've worked around this by running a function when a checkbox/radio button is clicked, that outlines the di开发者_开发知识库v around the button in black, so its not really a problem now.

But why does this happen?