That's because in recent releases, they would include a lot of binary blobs from other routers. Now they improved their GPL bundling a bit, so for example, the RT-AC66U binary blobs are no longer included with the RT-N66U archive. They also removed the RT-N12U binary blobs. They now also include the prebuilt binaries that had been missing from the past few GPL releases (I had to recover those missing files from their firmware binary releases to plug those holes).
Asuswrt-Merlin: Customized firmware for Asus routers
Github: github.com/RMerl - Twitter: RMerlinDev
See the sticky post for more info.