Because when Vieja are young all species tend to look similar, its hard to tell.
As an example
To me your pic #1 looks more like melanurus.

or even young bifasciatus than either of the two you mention.

and the 2nd pic looks more like a female breidhori

But of course they could just as well be zonatus or guttulata, or some hybrid combination (especially these days).
I have kept all of the above, juvie thru adult, and guess you need to wait until they are older and show adult colors to tell.