here first, folks. (At least, I think you did.) SM Stirling suggested that there is a tension between Poul Anderson's earlier "tough rubber bands" account of world lines, which would make changing the past very difficult, and Anderson's later incorporation of chaos theory, which, as his characters explain it, even implies that the past changes itself continually. I agree with Mr Stirling and also with regular correspondent Sean M. Brooks who suggests that this tension is a creative one. It provides the scope for at least one more Time Patrol novel.
Even on the earlier model, there were nexus points when it was easy to change the past. Thus, the idea of a "nexus" links the two accounts. What, more precisely, is a nexus point and can these points change somehow? There were already hints that all is not as it seems.
(i) Patrol recruits are told the truth - or as much of it as they can take.
(ii) Part of the truth that emerges is that the Patrol guards not only against time criminals but also against random fluctuations in space-time-energy. See here.
(iii) Reality is like waves on a sea; the probability-waves of underlying quantum chaos can change their rhythm.
(iv) Around times and places like Palestine, 69-70 AD, the continuum is vulnerable and reality is unstable even as far away as barbarian Germany. A branch of physics addresses this instability.
(v) Specifically, a text of Tacitus from another timeline shows up - somehow.
The Patrolman in the image is clearly pondering these issues.