Bigger firms like the nationals can afford to buy the extrusions up front and keep tens of thousands of pounds of it in stock, they can afford to pay the fabricators, the salesman, the surveyor the fitters all upfront before they even ask for payment from a customer, they can afford to wait for their money. Smaller regional companies and even smaller local companies have to buy their windows in from a larger company, every bodyalong that chain whats their deposit before proceeding with an order.
You only have to read the window fitters forum on FB to see the horror stories where their customers refused to pay once the job was finished even though the installation was fine and had no reason for witholding payment, often with an attitude of 'take me to court then', you can't remove your goods even though you stipulated goods remain the property of XYZ till paid for in full, that means nothing as you can't remove the doors or windows as you'll leave the house insecure, police won't get involved because its a civil matter so the fitter or firm is now out of pocket, which is why some smaller firms, sole traders LTD companies ask for at least the cost of what it costs them to get the frames, if the job goes south for whatever reason at least they haven't got a window/door supplier chasing them just because you might start playing silly buggers, they will of course take a hit on the labour though, some companies want 50% on order, 40% on start and 10% on completion subject to your satisfaction