I think it’s only less surprising because it happens more often, so that ‘everybody’ knows about it, not because it is inherently less strange.

As examples of merging, if you do

  s = sin(x)
  c = cos(x)

would you find it surprising if that generated a single sincos instruction if the CPU has one?

If you do

  typedef struct s { char a; char b; }
  s c;
  […]
  c.a = 3;
  c.b = 6;

do you find it surprising if the compiler generates a single instruction that is equivalent to (exact code will vary according to the CPU architecture, and this may need a further cast to avoid undefined behavior):

  *(short *)(&c) = 0x0603;