This is where I think it depends on the algorithm, because if Blizzard is honest about the drop rate, the pool shouldn’t make one bit of difference.
What a pool should theoretically do is guarantee a higher percentage of a specific legendary of a category.
That is:
Item rolls.
Determine it’s quality (ethereal, primal, ancient, legendary, etc.)
Determine the item.
So, if a legendary dropped once every 10 drops, but there were only 3 legendaries, after 10 drops, you would have a 1 in 3 chance of the legendary you’re after. But at level 70, the legendary chance still is once every 10 drops, it’s just that now, you have a 1 in 20 chance of getting the item.
If it went after determining the item first, with a smaller pool, the drop rate would be far higher than between ancient and primal. At which point, you’d create a new character and have someone power level you on T6 until you’re level 34 (which is around when the big boost of item pool comes into play).
I put in roughly only 2.5 hours into the PTR. I managed to get all 3 leveling on normal, as well as leveling post 70. I’ll agree, they dropped higher when leveling. But that could easily be explained by the following algorithm:
Item drops.
Is it primal? Impossible pre-70. Post 70, this might be hit instead.
Is it ancient? Impossible pre-70. Post 70, this might be hit instead.
Is it ethereal? Yes.
So, prior seasons, it would have been a legendary, as it would have skipped down to just being a legendary.
What you’re going to see is variance. I had a teammate when leveling up in the PTR. We leveled two characters each. I had all 3 Necro ethereals by Level 14, and all 3 DH ethereals by level 28. I never got a single duplicate for the first 3. My partner never got a single ethereal on either character while leveling up. And when I asked in another post, another player said they got multiple duplicates before they managed to acquire all 3. That’s certainly to be expected.
And then, we have to take into consideration that perhaps how the PTR works is a bit different than what we’re told will be the end result. Or maybe how leveling 1-70 is a bit different. It could be that 1-70, they have a 50% drop rate when it comes to legendaries.
Those are both in the realm of possibilities. But we won’t know until the season start.