Why K doesn't need copy from fixed column?

[qy3u]

While reading the code of the sha256 gadgets,I found that the K for calculateh_prime was assigned from a u32 table(ROUND_CONSTANTS) directly.

halo2/halo2_gadgets/src/sha256/table16/compression/subregion_main.rs

Lines 49 to 59 in 5678a50

	// H' = H + Ch(E, F, G) + s_upper_sigma_1(E) + K + W
	let h_prime = self.assign_h_prime(
	region,
	round_idx,
	h,
	ch,
	ch_neg,
	sigma_1,
	ROUND_CONSTANTS[round_idx.as_usize()],
	schedule_word,
	)?;

halo2/halo2_gadgets/src/sha256/table16/compression/compression_util.rs

Lines 748 to 755 in 5678a50

	// Assign k
	let k: [bool; 32] = i2lebsp(k.into());
	let k_lo: [bool; 16] = k[..16].try_into().unwrap();
	let k_hi: [bool; 16] = k[16..].try_into().unwrap();
	{
	AssignedBits::<16>::assign_bits(region, || "k_lo", a_6, row - 1, Value::known(k_lo))?;
	AssignedBits::<16>::assign_bits(region, || "k_hi", a_6, row, Value::known(k_hi))?;
	}

In my understanding, if you want to limit the value of a cell to some constant, you should copy it from a fixed column,but obvisously it's not the way here.So, Did i miss something or My understanding to constants in halo2 was wrong?