I think some systems do this in order to make sure the fans spin up. Typical case/CPU fans run full speed at 12v and the system knows that supplying a lower voltage will reduce the rotational speed.
When a system has been off for a while and everything is "cold" the lubricants in the fans is (typically) less viscous and a lower voltage will not be able to get the fan to start spinning because of the increased resistance.
To avoid this, for a few seconds at boot, the fans are run at the full 12v in order to ensure they spin up. From this moving the slight amount of heat created warms the lubricants, which makes them work better, and the fan will stay spinning at a lower voltage/rotational speed.