It's all to do with the fact that V1 is a maximum 'brakes on' speed if you decide to carry out an RTO. If the decision process starts at V1 and the decision is then made to RTO, the brakes on speed would be above V1.
Boeing state that the decision to carry out an RTO must be made prior to V1 and the first action (thrust levers closed, brakes on) must be carried out at, or prior to, V1.
Please cite a reference for "Boeing," since your claim is contrary to the governing FAA regulation. Note that the FAR already mandates that V1 is corrected for realistic reaction times, determined via flight testing:
§ 25.107 Takeoff speeds.
(a) V1must be established in relation to VEFas follows:
(1) VEFis the calibrated airspeed at which the critical engine is assumed to fail. VEF must be selected by the applicant, but may not be less than VMCG determined under §25.149(e).
(2) V1, in terms of calibrated airspeed, is selected by the applicant; however, V1may not be less than VEFplus the speed gained with critical engine inoperative during the time interval between the instant at which the critical engine is failed, and the instant at which the pilot recognizes and reacts to the engine failure, as indicated by the pilot's initiation of the first action (e.g., applying brakes, reducing thrust, deploying speed brakes) to stop the airplane during accelerate-stop tests.
For those who choose to call V1 3-5 knots or 3-5 seconds (per shazzmo) prior to actual V1, you risk that in the limiting case of accelerate and fly with an engine failure at LESS THAN V1 but AFTER the call, you will not get off the ground in time because of the additional acceleration time without the engine. If you KNOW what the limiting factor is in your specific circumstances, you MIGHT want to take that risk. If you do not KNOW the limiting factor for THIS runway/weather/TOGW case, you may want to reconsider...