Or Myriad Ways?

In Larry Niven's "All The Myriad Ways," timelines branch at every moment. If someone from timeline A visits timeline B, then timeline A branches into A1, A2, A3 etc in his absence. (I have just invented this letters and numbers system but I think that it expresses what Niven means.) When the traveler sets out to return to timeline A, then he arrives in A1, A2, A3 etc. Thus, he splits as well.

In Poul Anderson's Time Patrol universe, when Wanda sets out to return from the Pleistocene lodge to the twentieth century, why does she not arrive in the 1989 of the Danellian timeline and in 1989alpha and in the corresponding year of the Carthaginian timeline etc?

The only answer to this question is that that was not the kind of sf story or series that Anderson set out to write. His premise was: if time travelers can change the past, then there has to a police force to prevent such changes. Everything else that happens follows from that.