Blocks that are not near a difficulty change will always have the same difficulty.
No, the difficulty of a block is binomially distributed.
The
minimum difficulty required for a block to be valid is the thing that stays the same. Within that 2016-block window the
actual difficulty of various blocks varies above that threshold.
It would be less ambiguous if you said, "Successful pools do not build on the first block they hear; they build on the block with the highest work they hear."