…because that’s how our flash-kernel used to work (until the update that’s about to migrate to byzantium soon). The installed kernel was compressed, but u-boot expected an uncompressed image to boot from, so flash-kernel made an uncompressed copy, which also meant copying initrd (which doesn’t need decompression, so it got copied verbatim).
The new version gets rid of unversioned files, as new kernel packages now come with already decompressed images.