Signing payload is not the actually signing payload

[MoMannn]

In the polkadot examples for creating a raw transaction we have this line:

const signingPayload1 = construct.signingPayload(unsigned, { registry });

If we try and actually sign this payload via:

  const signature = await pair.sign(hexToU8a(signingPayload), {
    withType: true,
  });

The signature will be invalid. This is because this is not the actual signing payload.

If we go further by the code we can also do:

const signingPayload2 = registry
      .createType("ExtrinsicPayload", payload, {
        version: payload.version,
      })
      .toHex()

Which will return the same as payload above but its still not the actual raw payload we need to sign.

Doing:

 const signingPayload3 = u8aToHex(
      registry
        .createType("ExtrinsicPayload", payload, {
          version: payload.version,
        })
        .toU8a(true)
    )

Is what the actual signing payload is.

From my example the return of the above where:

signingPayload1: 0xa4040300fe747dadf0f62c7d1bac6988a156fdd41cf2d14ccfdc15e289512a7073bbf2660700e40b540295020c0000a9840f001b000000e143f23803ac50e8f6f8e62695d1ce9e4e1d68aa36c1cd2cfd15340213f3423e71e97c702816bfef0025cf7d067ec52fb8d345a5cbf2e69b81f7196ae80781bb00
signingPayload2: 0xa4040300fe747dadf0f62c7d1bac6988a156fdd41cf2d14ccfdc15e289512a7073bbf2660700e40b540295020c0000a9840f001b000000e143f23803ac50e8f6f8e62695d1ce9e4e1d68aa36c1cd2cfd15340213f3423e71e97c702816bfef0025cf7d067ec52fb8d345a5cbf2e69b81f7196ae80781bb00
signingPayload3: 0x040300fe747dadf0f62c7d1bac6988a156fdd41cf2d14ccfdc15e289512a7073bbf2660700e40b540295020c0000a9840f001b000000e143f23803ac50e8f6f8e62695d1ce9e4e1d68aa36c1cd2cfd15340213f3423e71e97c702816bfef0025cf7d067ec52fb8d345a5cbf2e69b81f7196ae80781bb00

Basically in first and second case 1 byte was added at the begining: a4.

Why is that? Why is it not documented anywhere? The naming is also extremely misleading when you are trying to actually generate and sign a raw extrinsic.