When I was the Area Administrator for North America (now Bernie Conway) the expiration subject came up. I fought for a ten year expiration and lost.
So far this disparity has not been a problem.
Note that IAAF has little interest in road races that are not big-time IAAF championship events, Olympic marathons, and AIMS races. The ordinary mom-and-pop races that are the backbone of racing are outside IAAF's area of interest.