TCP Flow-control questions that drives me mad.

Hi,

This is a capture of a TCP session when I was downloading 10Mb file from an FTP server. The server is has an ICW of 10x MSS, so this means that the server can initially send 10 segments without receiving an ACK from the client.

Being said that, every two segments has been ACKed in real-time, and the server has sent all the 10 segments to the client that has already ACKed all the segments. Now, the server waited from 15:40:31:864 to 15:40:31:901 for more than 50ms until it begin sending new segments.

I wonder, why the sever has paused sending more segments, although all the segments have already been ACKed? I know that it waited for the RTO value, but why would the server do this when all the ACKs have received?