This is because the Basic Noise Level is an LA10 (18-hour) whilst the segment total is an LAeq.
NoiseMap does the correction using the stipulated formula but does not show the amount of the correction.
Also, if you chose the 2005 conversion method, the segment totals are converted using the formula but the Combination totals are got from the cumulative LA10s and these are then converted. Different conversion formulae are used for motorway and non-motorway roads. This is as stipulated by the conversion procedure. So if you are using the 2005 procedure, you can't add all the segment totals to get the same as the printed combination total. (Although this does work with the 2003 procedure.) This is in essence the difference in the two procedures.
So don't try and get results for non-LA10 calculations by adding the segment totals, as this will not apply the formulae correctly. Use the Category Combination system for this.
