Tcp retransmission for some packets within 40 ms

Hi All, why are there so many retransmission with less than 40 millisecond.as per my understanding retransmission timer is generally 2*RTT so is it because of less RTT.I have collected this trace from hansang 2013 sharkfest collection on https://box.com/sharkfest2013 but i have uploaded it here, https://www.cloudshark.org/captures/d777543cd470 .