The first thing I would look at is whether the bullets are the correct diameter for your bore. How easy do they slide down? Can you shake it off the powder by holding the gun muzzle down and shaking it up and down hard? If so, you need to consider a slightly larger diameter bullet. Sometimes a difference of .001" is all it takes. NE does have at least a couple of sizes for their bullets. Getting the correctsize will help in sealing the bore and getting the best performance out of them.

At 70 grains 777 your velocity is probably about 1200 fps and at that speed the total drop from bore axis is around 3" at 50 yards. The difference your seeing is probably a combination of the point of exit in the barrel vibration (low point of vibration) and the sights (or sightingtechnique)you have. At 90 grains 777 my guns push that bullet between 1450 and 1500 fpswhich givesa total drop of about a 1/4" at 50 yards.

Are you seeing about a 2.75" difference in point of impact between those two loadings without adjusting the sights?

Either way you will probably want to look into the lower front sight (so the rear sight doesn't get too high, unless you need it higher) to help get the impact up where it needs to be. If you are using a 6 o'clock hold, that can cause the impact to be lower than a dead on hold. I personally like to use a peep sight when I'm not using a scope, it just seems to work better for me than regular iron sights.