clock buffer vs normal buffer
When we use clock buffer, its purpose is to equal duty cycle for all f/f.
whereas, when we use normal buffer, its purpose is to meet timing.
The timescale of normal buffer is smaller than clock buffer.
Normal buffer is related with set-up/hole timing violation.
The timescale you can fix using normal buffer is too small.
Comparing with that, clock buffer's timescale is larger.
As you can guess in design compiler constraint, clock uncertainty is 0.2 or 0.3 ns.
If your design's clock uncertainty meets the above range, it's okay.
So clock buffer and normal buffer have diffierent timescale to handle.
It's not fact, but i'm just guessing with my back ground.
Thank you!