[CentOS] Scp and SFTP speed much difference???

we have CENTOS 5.X on server A and Server B.  Both servers have Gigabit network card connect to same switch.

I test network speed from Server A to Server B and found speed much difference:

SFTP:   42 MB/Sec

SCP:    20 MB/sec

SCP command:  scp -Crp -o CompressionLevel=1 user1 at ServerB:/TMP/File1 .

Anyone know how to improve SCP speed?