When buying the items in your Wanted List in the Buy Page, BrickLink will
automatically account for each item’s Have Quantity and look for the remaining
Wanted Quantity. https://www.bricklink.com/help.asp?helpID=1
That doesn't work for wanted items in a store. Example can be seen here:
https://flic.kr/p/EAFcDz
[...] Move the parts to 'Have'. When buying the items in your Wanted List in the Buy Page, BrickLink will automatically account for each item’s Have Quantity and look [...] (92 months ago, Nov 13, 2016, to Suggestions)