As others have said its the variation in components & equipment that you're seeing. Sometimes it can be quite large.
My usual advice is to look a little deeper than just the load tables. There is usually some kind of preface or intro to the load. I look at this first & allopcate a priority from several manuals, giving preference for the closest match for components & equipment to what I'm actually loading.
AS an example I was working up some loads for a 6" revolver. One of the sources had highger velocities than any of the others, looking at the equipment used it had a 10" pressure barrel in a universal reciever. I ignored that data.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”