What does anachronizing mean here?

Context:

Tacitus is in error because he refers to Pilate as a "procurator" when in reality Pilate was a prefect. This means that he is unreliable, or that he probably did not consult written documents.

This objection is also favored by Wells [Well.DidJ, 10; Well.HistEv, 16; Well.JesL, 42]. However, as Chilton and Evans remark, "(t)his 'error' should not be taken as evidence that Tacitus' information is faulty" [ChilEv.Stud, 465]. Two reasons may be cited for this:

Evidence indicates that there was a certain fluidity in the usage of these terms.
Tacitus may have been anachronizing on purpose.
We should first consider the difference between these two titles. A procurator, as the word implies, was a financial administrator who acted as the emperor's personal agent. A prefect was a military official.

More:
http://www.tektonics.org/jesusexist/tacitus.html