The groove is wide to accomodate expansion so the boards can float. Bolted down tight the bed would buckle with a change in humidity. The boards expand much more width-wise than they do length wise.
The machine shop at my old job had a wood block floor. One night the roof started leaking, the blocks swelled, and there was a 12" high hump from the blocks expanding. Surprising to see. It dried out and the blocks went right back into position.
