Veneer is sliced from the log in a variety of methods- Flat, Quarter, Rift, Rotary,etc. I'm sure each method of slicing has challenges to maintaining uniform thickness.
It is important to understand that veneer is sliced from the log with a blade that's often over 12' long. While they're able to get a pretty uniform veneer thickness, uniform in woodwork and metalwork are two different things. My thin veneer may vary from .016 to .024 depending on the sheet. I try to order complete flitches of several thousand square feet of raw veneer at a time, hoping to keep the thickness as consistent as possible. My current batch of thin is mostly around .019.
Best of luck,