Those are all short intervals, where it's easy for your anaerobic capacity to skew the results upward.

And using NP for an hour race to estimate FTP really is worthless. It doesn't need to be all that inaccurate to be so, either. For a 300W FTP, z3-z5 are what? About 30-35W each? If your FTP estimate is 10% off, you're going to be completely in the wrong zone when training. If the FTP is set anything over 10% too low, z5 VO2max workouts become z3/4 sweet spot workouts. And you'll only do 5-8 minute intervals - at a power level you should be able to hold for over an hour without too much difficulty. If your FTP is set just 10% too high, you won't be able to complete any real z4 or z5 workouts at the prescribed power levels.

The fact that you're only doing 2 ten-minute intervals is informative. You can't do four or six of them at those power levels because your anaerobic capacity gets depleted and your aerobic capacity (FTP) isn't high enough to sustain you for that many intervals.