Your volume is not a cube. Chimera by default generates molmap mrcs in a minimal box, which can have odd dimensions. You can correct this by resampling on the grid of the original map - e.g. vop resample #map1 ongrid #map2 - or by using molmap with the ongrid option - e.g. molmap sel 20 ongrid #mymap
Cheers
Oli