The simple answer is because it is its own signal, so it can be a different length. There is no condition that would force the noise bin data to be the same length as the sampled signal.
When the noise bins are added to the sampled signal, for example setting Convert Noise Bins to true, then during the conversion the noise bins signal will be converted to a sampled signal with the same length. I am not sure how else to explain it, but let me know if you have any questions.
Cheers!