what's the difference between populating via databind and via dropdownlist.items.add(new listitem)

Question

I have a dropdownlist that list or major models, when selected it filters down to specific model on my 2nd dropdownlist, the problem is when I select on the 2nd dropdownlist the first time I get the correct selected value , the 2nd time I get the same model like it didn't refreshed. To illustrate:

1st dropdownlist

General Model 1 General Model 2 General Model 3 - Selected

2nd dropdownlist (populated based on 1st selection)

specific model 1 specific model 2 specific model 3 - selected

button - show result - selected model 3

now, this time select specific model 1, result- selected model 3 , this is my problem for some reason it's not returning the new selection.

anybody solved this particular issue? Any advise is much appreciated.

cheers!