I am thinking of replacing the rims on my wheels. They are not great quality wheels but it may be worth doing if I can get rims with the same erd and replace using the same spokes. The old rims are Mavic X139 for rim brakes, not the disc brake version. They are quite old and haven't been produced for a while so I am finding it difficult to get a reliable erd for them.

The spocalc spreadsheet lists them as 548. This would be ideal as I can get sun cr18 rims cheap which are also 548 according to the sun website. However, spocalc list the cr18 as 544 instead of 548. Hence I am now not sure whether the spocalc x139 erd is accurate Is there a reason why the erd for the cr18 on spocalc is different to the sun site other than spocalc simply being wrong?