Why largest-first matters
Variable Length Subnet Masking lets each subnet be only as big as it needs to be, but the order you allocate in decides whether the blocks pack neatly. Every block has to start on a multiple of its own size, so laying a large block after several small ones leaves gaps that nothing can use. Work down from the biggest department and the space fills cleanly, which is exactly what the meter in this lab is showing you.
Learn the idea behind the lab
This lab is the practice; Subnetting & CIDR is the explanation. The Subnet Calculator will check any single block for you.