Just because something is accepted as an historical consensus doesn't necessarily make it true. Textbooks, whilst more balanced than monographs, will still be presenting the author's point of view. Every historian has a point of view- hence historiography, examining these- and textbooks will still incorporate these. The author has to simplify to an extent- they can't say that "Historian X thinks this, Y this, whereas Z thinks the other" to every single statement so they generally make generalisations.
They'll also be writing for an audience of students, some of whom may not be particularly advanced or capable of complex analysis.
Yes, authors are generally aware of this, but still the authors of textbooks have points of view and they can be contrasted with others. Ergo whereas they're not "biased" as such (HATE that word!) they are still influenced by the author's point of view.