It's because he's consciously changing it, not that it's happening by itself. Now, of course it does change by itself over the years too so, ADD that to the fact he's purposely altering it himself for a new style. And that explains why it's so drastic.
He even said in 2002, for a TES interview that... 'if you sound the same as your last album - what's the point' (paraphrase).
On
The Slim Shady LP he was altering his own voice as he was in character. Even if he didn't fully understand why at the time as he says he looks back like "why was I rapping like that

?" but, it's because he was in this slap-happy character obviously.
Then on
The Marshall Mathers LP he went more with his natural tone, as it was more realistic.
Then on T
he Eminem Show the flows were hyper-focused and, more rock-tinged, layered and louder. More shouting and emphasis in every sense.
Then on
Encore, people think he sounded 'lazy', which... I'm sure he was in spots but I also think he purposely employed a more laid back style to separate it from
The Eminem Show. Because, if you consider the amount of high-octane energy in
The Eminem Show, how would he possibly top that? he needed to come 'under' it so to speak. Come with laid back, more tricky flows.
Which is what you see on say "Evil Deeds" and "My 1st Single".
But he just experiments a lot, pushes his tones and vocal extremities to the limit and by doing that... you create new barriers for your voice. I've learned that over the years even in doing silly voices and impressions. After a while you can slip into tones you never could before, much more easily.
Your brain / throat gets trained to it. Just like singing. But, it's just a different art. But I agree, it's incredible. The fact he's on his seventh album and is still able to surprise you with his voice or bring a new style you haven't quite heard before is something I've never seen from a rapper.