How is this possible? There can be a number of contributing factors.
The data in the reloading manual was tested by the manufacturer to be safe using the powder, case, primer, bullet, and importantly - the firearm specified.
If you change any of these parameters, the pressure developed by the cartridge will change.
If you change more than one parameter, you run the risk of 'stacking' a number of slight increases in pressure that add up to an unsafe level. ]