Does TCP stream graph show some data points incorrectly?

Using version 1.6.4, I want to rely on TCP stream graph to chart and summarize RTT for ACKs. I capture data because I'm suspecting some network delay is impacting a unicast HD Video on Demand application I'm testing. This traffic is delivered via HTTP over TCP. Most data points plot in the graph around 30ms value which is okay for my application and normal for my network environment.

A few plotted values on the graph are at high (problematic) values between 55ms and 65ms. Yet when I click on any of these values in the graph, and then go to the selected frame, and the RTT value shown within the packet details, is low/normal value. When I take the difference between the time stamps of the ACK and the packet the ACK was for, the answer does correlate with the timee stamps.

Is there a known bug with the TCP stream graphing so RTT values do not always display correctly????