usually due to third party software running in the browser, probably Epson or HP toolbar or print utility
few web pages are designed to print A4, you usually find the side is cropped. As a result some of the printer companies provide browser plugins which automatically resize the job to the paper size.
All you can do is see what software is running within the browser, check the brower scale settings, and also check in the actual printer setting as to whether you have a "scale to fit" or "shrink to fit" setting enabled
Its worth pointing out that IE is more of a pain than Mozilla when printing web pages: Mozilla sometimes rescales pages and fonts itself, IE never does