I don't know about Latin specifically, but the use and meaning of punctuation like the colon have been changing over the years. There haven't always been clear rules, nor are there universal and fixed rules till this day. If you look at English from the last few centuries, you'll see differing usages through time. By modern standards, it can be really strange at times, varying within a period and even within the works of a single authors. It's only recently that we've attempted to truly standardize punctuation to meet stylistic guidelines.
I imagine all of this applies to Latin texts as well as vernacular ones, considering that modern editors are creating these "critical editions". Editors necessarily have to make choices about how they are going to present the text, and if they are editing something that is originally unpunctuated, they will undoubtedly follow their own judgments, education, and the prevailing style of the time.
Horae quidem cedunt et dies et menses et anni, nec praeteritum tempus umquam revertitur nec quid sequatur sciri potest. Quod cuique temporis ad vivendum datur, eo debet esse contentus. --Cicero, De Senectute