I think the others have pretty much covered it. I can just add that I have a Copley delrin flute, and if anything I would say that it is tuned a little high of A=440 hz with the slide all the way in. I tend to blow a little flatter than average and even I need to extend the slide a few mm to get it playing at A=440 hz. Aside from that, it is very well in tune and easy to play. In my experience Dave is also very consistent in his flute making. So this makes me suspect that the problem you have is most likely either (a) something to do with the tuner you are using to take the measurement, (b) an extremely low temperature in the environment you are in when you are measuring, perhaps also combined with a cold flute, or (c) you are over-covering the embouchure very significantly. Given how low the tuning you describe is, I think (a) is most likely, then (b), and then (c). Of course, it could also be a combination.
I play a 100 year old wooden Haynes Boehm flute pitched at 435. I have had to work hard to bring the pitch up to 440 , especially on low C - E. I handed the flute to a professional classical fluter and she ran it through 3 scales at 440 without problems. If you develop your embouchure to that level you can adjust as you play to bring any note in tune. She also showed me to push the flute out more with my right hand as a way of raising the pitch. Remember blowing down into the hole to get a deep sound will lower the pitch.
For many years I played an original c1830 Rudall & Rose and then for many more years an original c1860 Koehler & Son (London) Pratten model and neither had Bottom Ds which were flat enough to be a problem. No lipping required, just supporting the note sufficiently.
BTW I used the footjoint keys regularly on both flutes and those notes were in tune as well.
I’ve seen players who have the headjoint turned in between the 1 o’clock and 2 o’clock position, so they’re blowing down into the blowhole rather than across it, which makes the flute play much flatter.
I play with the far edge of the blowhole in line with the fingerholes, a bit turned in compared to Boehm players.
I’ve only owned one flute which was too flat with the head pushed home, a Ralph Sweet Low C flute. It was very flat, around a quartertone. The cork was in the proper position for the octaves etc. I sent it to have the top of the body chopped and the tenon re-turned which fixed the problem.
They’re useful for telling you the pitch of the note you’re playing (which is all they’re designed to do) but they won’t magically make you play in tune, true enough.
They can develop more in-tune playing if you practice with one… though who wants to practice watching a needle?
Since the point is to develop the ear what I used to do was to set a tuner playing a tone, D, and practiced playing up and down the scale making every note in tune with that drone-note. Being a Highland piper before I was a fluteplayer it was a satisfying way to practice.
EDIT: sorry, that’s “Band Tools”
PanCelticPiper, I did say that they are useful for finding a starting pitch like G or A. In my personal opinion, rather than using tuners and metronomes to practise pitch or timing it’s much more useful to play along with recordings of top players. At least that way you’re practicing pitch, timing, lift, the tunes themselves and what it it may be like playing with other people… but that’s another can of worms entirely!
What I supposed could be called the Gestalt approach, which in the old days was probably how everybody learned.
I find that sometimes beginners are overwhelmed by having everything hit them at once, and that they progress faster by compartmentalising and focusing on one aspect at a time.
The Highland pipes are an example of that. When you learn the uilleann pipes, the Northumbrian pipes, the Bulgarian pipes, and probably just about every species of bagpipe you’re learning the blowing of the bag and the fingering of the chanter at once. With bellows pipes you have yet another thing to figure out from the get-go. But Highland pipe teachers at some point felt that the learning of the fingering needed to be separated from the learning of the blowing of the pipes themselves. (In like manner many flute teachers have the beginner learn blowing with just the headjoint.)