Because it is a natural wood product and is subject to the envirinment.
NOFMA/NWFA clearly state that due to the dimensional change due to exposure to environmental conditions, milling tolerances are only valid at the time of original milling at the factory. You cannot measure the width or thickness of a board after that time and compare to established milling tolerances. It is not a valid measurement.
For the purposes of determining acceptability at the time of installation (assuming proper acclimation under controlled conditions), the installer must use the tolerances for visible gapping/overwood. To do this, the product must be assembled tightly according to good installation practices (not less than 6” between the ends of adjacent boards, no two adjacent boards can be the same length, etc.). Assuming proper acclimation and good assembly/installation practices, Bellawood gap/overwood tolerances are: VISIBLE GAPS: Select Grade: ≤0.025” VISIBLE OVERWOOD: Select Grade: ≤0.020”
Please note, any visible gaps/overwood less than the maximum tolerances above are not considered defects and do not apply to the 5%-10% defect/waste allowance based on the species guidelines. Pieces that exhibit gaps/overwood in excess of these tolerances should be culled at the time of installation. Installation constitutes acceptance. Only those visible gaps/overwood in excess of these tolerances apply to the 5-10% defect/waste allowance.
Bellawood gap/overwood tolerances are going to be within basic milling standards.
Keep in mind that milling standards are based on NWFA/NOFMA standards which are National or Domestic floors and do not take into account the milling standards of exotic woods. While most manufacturers will use this standard for exotic woods there is no set standard by NWFA or NOFMA for these tolerances.
In addition these tolerances are based on domestic plain sawn woods. It does not take into account the mix of ¼ sawn and rift sawn material that are included in the wood.
Answered on 4/25/2012 by cc-2