Dropdown menu respects filter

Hello guys,

I have a normal validation dropdown menu/list. (say it is based on the cells B1:B20)
When i use a filter and the results are cell B1,B5 and B8 the dropdown menu still shows cel B1:B20
How can i make it only show cell B1,B5 and B8? I hope somebody can help me!

Gr.
Blue

Possible...
please attache sample data sheet.

Regards,
Suhas