I was writing a python module that reads a gnumeric file and just writes it back again. I noticed that everything worked except the column width wasn't preserved. The python module changed the order in which XML attributes are written in the XML file. If I use a text editor to change this part of the XML file back, then it works again:

      <gnm:Cols DefaultSizePts="48">
        <gnm:ColInfo HardSize="1" No="0" Unit="114" />
        <gnm:ColInfo HardSize="1" No="1" Unit="394.5" />
        <gnm:ColInfo HardSize="1" No="2" Unit="73.5" />
        <gnm:ColInfo HardSize="1" No="3" Unit="51.75" />
        <gnm:ColInfo HardSize="1" No="4" Unit="31.5" />
        <gnm:ColInfo No="5" Unit="27.75" />
      </gnm:Cols>

was changed into:

       <gnm:Cols DefaultSizePts="48">
        <gnm:ColInfo No="0" Unit="114" HardSize="1"/>
        <gnm:ColInfo No="1" Unit="394.5" HardSize="1"/>
        <gnm:ColInfo No="2" Unit="73.5" HardSize="1"/>
        <gnm:ColInfo No="3" Unit="51.75" HardSize="1"/>
        <gnm:ColInfo No="4" Unit="31.5" HardSize="1"/>
        <gnm:ColInfo No="5" Unit="27.75"/>
      </gnm:Cols>

On irc this behavior was confirmed, it seems necessary that No comes first. The XML spec however says attributes do not need to be ordered.