The custom label in structual rule show a wrong value after cutting the model?

I defined two custom labels  in structual rule, the 1st one named Length with the attribute of  "Length", the 2nd named SectionName with the attribute of  "SectionName".

Then I draw a simple beam to test the rule, the result shows that this two labels take effect but the value of the 2nd label show a wrong value, why? I 've checked the datagroup data of the beam, it's property of Length is existent.