Tell you in store "no stock" but on line you can collect no problem... why?

I went to Leeroy Merlin to get some Vinyl Sheeting, we liked the black the white checkered on, but they only had a piece in stock, liked the wood finish one as 2nd choice, also just a piece left, the guy that was helping us even checked "on da computa" and it showed no stock. So we went back to the car, went online and ordered 12 meters of the black and white vinyl sheeting, went to collections and picked it up, why was I told "no stock as per da computa" and shown away, why did i have to order online to get the stock that I asked for? How much money is the store not loosing due to staff not knowing what stock you have on hand even when they check "on da computa" This is really unacceptable that i have to resort to online ordering when i could of just got it at the store. Order #*** - 55885 Order #*** - 55891