The proposals use different installation scopes
An asphalt tear-off and a metal overlay are not merely different coverings at different prices; they also offer different access to underlying conditions. Align the scope before making the cost comparison. Ask what each bid includes for deck inspection, old material, flashing and concealed damage so that a lower total is not mistaken for equivalent work.
Notice next: Create a side-by-side list of included work and ask bidders to explain each difference.