The Linux kernel will keep going after the "TRAILER!!!" entry in the initramfs. This is useful in the context of initramfs because you can concatenate them together - a trivial way to add more files to the init ramdisk or even overwrite existing ones. You can even concat gz-compressed cpio initramfs files which causes a similar problem finding the uncompressed size because the last 4 bytes will only show the size of the last gzip section (try with gzip -l).