I have been given several reasons by experienced reloaders but the one that has proven itself is that the onformation in reloading manuals will have come from different conditions (barrels, chambers, brass, environmental conditions, etc...) These conditions will reflect differently on the load data represented. I have three manuals (sierra, nosler, and lee) and am looking to get another two at least (any suggestions?). One thing I have found is that your rifle may be very close to the load data in one manual but very different than another, then when you switch one of the variables (bullets mostly) the information may be closer to a different manual. If you are just looking for a step by step procedure they should all have similar basics, however if you are lookiing at getting into benchrest type reloading you may need to out source your information to a board such as this one. I have found that the basics can be covered in the books but the experience/information that is relayed through these boards is second to none.