sweet, i was right
anything that can happen with a disc i've probably seen happen with at least 10 students and probably experimented with myself as well. it's possible to throw ~430-450' with that position but requires superb timing and probably can only be done with really understable plastic. i had a period where i threw that way and was able to get 425'+ with stuff like beat in pro orcs and z flashes... starting them slightly nose up but using OAT to get the nose down and to flatten them from a hyzer. it's not an ideal way to throw...
Mrscoopa is most likely correct on what happened with you when you pulled tight.
i can elaborate on it a bit more:
when you kept it tight it's likely that you brought your elbow "around" your body once the disc reached the power zone rather than letting your elbow getting out in front of you and extending. the result was a few things: you never got a lot of power on the throw, your weight was probably back, and you likely had a wrist roll under (due to the weight back).
trying a few throws starting with the disc almost touching your stomach should probably help that a bit. your initial motion should be forwards and not a rotational motion. that will probably help get things on track.