The window is narrower than it looks
A site running two shifts may appear to have several hours between them, and in practice have twenty minutes once handover, cleaning down of equipment and the early arrivals for the next shift are accounted for. A provider quoting from a stated finishing time will build a schedule around a window that does not exist. The result is either an incomplete clean or a cleaner working around people, neither of which was priced.
Ask what the provider believes the window is and where they got that figure. If it came from the walkthrough conversation, check it against reality with somebody who works the changeover. Production supervisors and shift leaders generally have a far more accurate picture than the office does, and half an hour spent with one of them before quotes go out prevents a schedule that will not survive its first month.
