The TCP window is calculated after some sending activity. It reflects the amount of data that can be transmitted in one shot without hurting other TCP flows at the bottleneck.

Reusing TCP windows after a while does not reflect the actual state of the network as things would have changed. In fact, there have been proposals about CWND reset[1] that adress this issue.

[2] http://www.ietf.org/rfc/rfc2861.txt