It's because you are trying to print documents which are formatted for 'Letter' paper rather than A4. This could be because soemone has sent you a document in this format, but the most likely explanation is that you are using Word and this has been set as the default page size for new documents.
Try this: Open a new blank document in Word.
Click File/Page setup
Select the Paper tab
Check that the Paper size is set to A4
If it's not:
Change it to A4
Click OK
Click File/Save as
Change the 'Save as type' to Document Template
You should see a file "Normal.dot'
Overwrite this file.
This will ensure that any new documents are created for A4 pages.
For any existing documents, you will need to load them, change the paper size (as above) and then just resave them.
It's also worth checking your printer manual - some printers have the option to automatically scale Letter documents to fit on A4 paper.