please accept my excuses for being lazy; a closer look in fact is over due.
So here we go:
The "Flying Tourbillon" in fact exists in 3 variants: Titanium, Rose Gold and Platinum. The TI version is depth proof to 3.000m, the other version depth proof to 1.000m.
To explain the difference, a closer look at what is not apparent:
The Titanium version does come with a massive case, while both precious metal variants do include a sort of "inner shielding".
This because of the specific properties of gold and platinum, which are less hard than Titanium and hence are not capable to withstand a great depth by nature.
The inner case does change this, with the result of 1.000m in depth rating.
Of course it would have been possible to achieve the same by widening the case walls; just that an even thicker and wider case wouldn´t result in a very wearable watch. The whole thing isn´t exactly small as it is right now and to achieve a depth rating of 1.000m with a massive gold case will end up in a diameter of close to 50mm. Not to speak of the weight - it´s more than 200g right now (on strap) and a larger/more heavy case probably wouldn´t be wearable at all unless the owner happens to be the incredible Hulk
In short, the depth rating goes back to the specific properties of precious metals. In some way, it´s a bit of a compromise; personally, i find the inner case quite a clever solution and a depth rating of 1.000m probably sufficient for most opportunities. After all, chances are low to test it on the wrist
Hope this explains a bit?