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.
Different carriers cater for this in different ways. Some call V1 slightly early, others will always use the WET V1 figure, even on a bare/dry runway. Ours just emphasises that the decision to RTO has to be made prior to V1 and that the RTO must be commenced by V1.
As an airmanship point it is always important to know what your limiting performance criteria is. Eg, if you are climb limited you will have some spare stopping performance in hand but if you are field length limited then a V1 RTO will leave you in very close proximity to the wrong end of the runway.