ok, answe this simple question..... why did they have staggered launches back in the day?
If you truly understand why, then you would know why its not an unfair way to make a comparison.
Because it's easier to prepare launches by doing one region at a time. The PS4 had a staggered launch as well, in case you forgot.
Aligning the launches by region and then adding them up to global sales makes the most sense to do comparisons. Otherwise you are looking at nonsense like the early PSP vs. PSV comparisons that had people go "Oh, the Vita isn't doing bad. It's tracking at a similar pace.", when in reality Vita's worldwide sales were compared to only the PSP's Japanese sales, because the PSP had a differently staggered launch. Anyone who used the method of aligning launches by region could see how awful the Vita was really doing, because that method painted a more accurate picture.
Likewise, in the case of the PS2 vs. PS4, your comparison shows the PS4 ahead which implies that the PS4 has a realistic shot at matching the PS2. Aligning launches by region, however, shows that the PS4 has already fallen behind by a significant margin which puts the PS4's chances to match the PS2 at very slim to none. The latter method is a more accurate reflection of the situation and allows better analyses as well as predictions.