Yeah but, Em is consciously changing it. That's why.
Jay sounds the same as he doesn't make much effort to sound different. He may switch up flows but he doesn't consciously force a tone-change and... no rapper I've ever heard has put the same effort and focus on vocal emphasis as Eminem.
Like Game said, Em's like Eddie Murphy in the studio and that poet that praised him also commented on his verbal energy. There's something about the elasticity and clarity, and sharpness of his voice, but I don't think it's entirely natural. I think he's worked on it.
And, every album he's purposely sounding different. So, where as with most rappers all you'll hear is the natural age of their voice which can often not be very noticeable at all, Em is making an effort to change his entire tone for an album. And, I'd put a bet on more than we think. As in, I bet he spends a good while finding a new tone and honing it.
If you listen to even his growl and angry voices for example, they're so... powerful and... I dunno. There's something incredibly sharp and honed about his voice(s). It's practice.