sure, because they are measuring progress towards completion, rather then the ability to fly i.e. if a finished plane weights 10,000 lbs all put together, then

sure, because they are measuring progress towards completion, rather then the ability to fly i.e. if a finished plane weights 10,000 lbs all put together, then a in-construction plane currently at 1000 lbs is clearly further along towards completion then one at 1 lbs