why rsync --sparse create much larger file than the source?

[braindevices]

I try to rsync between 2 local disks, thus there is no compression. the sparse file has apparent size of 50GB, actual size of 16GB. I tried rsync -a -P --sparse, it did generate sparse file on destination at the end. However, it is not as sparse as the source file. The generated file is 27GB instead of 16GB. I can use fallocate -d <filename> to shrink it more. It became around 18GB. I wonder why rsync cannot create sparse file as cp -a --sparse? cp can create sparse file with identical size as source.

rsync 3.1.3