I found a workaround that seems to work for me, by extending the selection to start of the 1st line and to the end of the last line before retrieve the selection lines count.
Code:
With Selection
.StartOf Unit:=wdLine, Extend:=wdExtend
.EndOf Unit:=wdLine, Extend:=wdExtend
iLins = .Range.ComputeStatistics(wdStatisticLines)
End With