Here's enough info to drive ya' nuts, Toot!
Starting with measuring technique ... assuming AP flour and 1 cup:
1) If you scoop out of the bag (using the measuring cup) and scrape it off on the side of the bag it's about 5+ oz
2) If you scoop out of a cannister (using the measuring cup) and scrape level with a straight edge, about 5 oz
3) If you scoop the flour out of bag/cannister with a spoon and spoon into the measuring cup and level with a straight edge - 4.5 - 4.7 oz
4) If you scoop the flour out of bag/cannister with a spoon and sprinkle it into the measuring cup and level with a straight edge - 4.0 - 4.5 oz
5) If you sift the flour into the measuring cup - 3-3.5 oz
6) If you shake, bump or thump the cup to level it - it will weigh more than if you don't.
There is also a difference between regions. AP flour in Canada and the Northern US regional flours tend to weight more than National US brands which weigh more than Southen US regional brands. Shirley Corriher mentions this in her book Cookwise.
The type of flour also makes a difference in weight per unit volume.
Glad I could help simplify this for you!