Are you saying that the new storm windows completely fall through the window opening, including the flange which would otherwise overlap the outside window frame so that it could be screwed firmly in place?
Regardless of who is in error, I would contact Lowes and see if they would not exchange for the properly sized windows. I did not try to download any supporting literature (did not see any at a quick glance) but IMO the information on the web page is not very complete. It only talks about dimensions of the opening, not other important details like how much does the attachment flange overlap, how thick are the windows, etc.