If you are truly exempt, then "40 hours a week" has no legal meaning to you. The law does not care whether an exempt employee works over 40 hours, under 40 hours, or exactly 40 hours. The law, however, gives the employer the right to determine what hours are needed.
The only thing an exempt employee is exempt from, is overtime pay. He or she is not exempt from following company policy regarding hours worked, time due in, time allowed to leave, or anything else except overtime pay. It is up to the employer how much freedom the employee is given with regards to their hours.
Not all jobs necessarily require the same hours; therefore not all exempt employees need to be treated identically.