Rsync --checksum does not catch changed files

I am transferring files from one linux computer to a mac. The coection sometimes drops out and I restart again. This is usually fine, but I have to check everything for corruption as a result.

rsync -avc takes its time to to run the checksum, but then declares everything in order. I have double checked this with md5sum and found they are different. Substantially different. It seems that either rsync's checksum is too simple (crc?) or just not working.

linux rsync is 3.1.1 and on os x i have tried the native 2.6.9 and homebrew installed 3.1.2

any ideas?