There are always, and always will be, differing opinions on the ideal temperatures.
Here are but a few of the things which contribute to variance:
1. Mold material- iron, aluminum, brass? Each material has differing properties of heat retention, thus affecting the casting pace used.
2. Size of cavities.
3. Number of cavities.
4. Alloy being used- one person's alloy is probably not identical to another person's alloy, even though both are using "lino" or "WW".
5. Casting pace of the individual. Somebody casting at a faster pace keeps the mold hotter.
6. Casting method of individual- ladle or bottom-pour? How much "drop" from pour spout to mold?
7.Calibration of equipment, especially thermometers. Your thermometer may read 650, another may read 675. Which one do you quote?
8. And on and on and on...
Don't sweat it too much. Personally, I always cast at lowest temp possible for the combination at hand. But in the end it's all about the end result- quality bullets.
Most of the time the higher temps are quoted to make up for deficiencies elsewhere in the casting chain. If you feel the temp for you is too high, evaluate all factors involved and try lower.
Good luck,
Rob