Vijaya,
If i understood your question correctly then
Usually in your Valuation cost will reduced with Movement type 531 and increased with movement type 532.
Say example Your Material X=2000
By Product Y per Kg@100=800
Overhead=100
with Movement Type 531 So the value will be like this X (2000)- Y (800)+ OH 100=1100
Movement type 532 so with reversal it will be different X 2000+Y800+OH100=2900.
If your question is why By product value is different then check your settings for Freight posting ( movement type (531 or 532) or addictive cost configuration.
Best Regards
Hrusikesh