Here is a part of the law:
----------
(1) Whenever a child who is less than sixteen years of age is being transported in a motor vehicle that is in operation and that is required by RCW 46.37.510 to be equipped with a safety belt system in a passenger seating position, or is being transported in a neighborhood electric vehicle that is in operation, the driver of the vehicle shall keep the child properly restrained as follows:
(a) If the child is less than six years old and/or sixty pounds and the passenger seating position equipped with a safety belt system allows sufficient space for installation, then the child will be restrained in a child restraint system that complies with standards of the United States department of transportation and that is secured in the vehicle in accordance with instructions of the manufacturer of the child restraint system;
(b) If the child is less than one year of age or weighs less than twenty pounds, the child shall be properly restrained in a rear-facing infant seat;
(c) If the child is more than one but less than four years of age or weighs less than forty pounds but at least twenty pounds, the child shall be properly restrained in a forward facing child safety seat restraint system;
(d) If the child is less than six but at least four years of age or weighs less than sixty pounds but at least forty pounds, the child shall be properly restrained in a child booster seat;
(e) If the child is six years of age or older or weighs more than sixty pounds, the child shall be properly restrained with the motor vehicle's safety belt properly adjusted and fastened around the child's body or an appropriately fitting booster seat; and
------------
The way the law reads:
If the child is less than one year of age or weighs less than twenty pounds, the child shall be properly restrained in a rear-facing infant seat
can be seen as
if A or B, then C
If A is true or B is true, then C must occur, so if a child is less then one year of age (A) or less then 20 pounds (B), then the child shall be properly restrained in a rear-facing infant seat (C). You don't have to have both A and B true for C to happen. If it had been "less then one year AND 20 pounds" then the law would be much worse as then both conditions would have to occur for the child forced to be rear facing.