Arrow spine question

[mountain lion]

I know, another arrow question! Lol here is the question, ok now I shoot the mfx classic 500's and they fly like darts. Now I grabbed a 400 one day by mistake and when I shot, it was great left and right but It was way nock low!! I mean way low, I had the nock point at like and 1 1/4 before the shaft was flying straight. Now could anybody tell me why that is?

[BAK]

Explain what you mean by that.  By what measure.

[mountain lion]

When I shoot the 500 arrow out of my bow, which is the correct size it doesn't fly nock high or low or left or right. But when I shoot the 400 out of the same bow with the same 75 gr insert and 125 gr tip as the 500. The nock left and right is good but it shoots way nock low, like its sticking in the target at like a 45 degree angle. I just can't figure it out