You dont. You can go off one or the other but first of all blueprints may not be 100% accurate. Let take LA for example, firstly Pierce custom builds there engines using designs provided by LAFD. So in this case blueprints may not be exactly the same as the standard Pierce Arrow XT(assumption). Now its also possible that the department has made after market customizations. Using LA and Pierce as an example, notice the steel box above the hose bed in this picture?
Pierce does not deliver the engines with these boxes. LAFD uses these to store things like sleeping bags and such that may come in handy during times when there working brush fires for days at a time. Now besides that, which is in part speculation, with real photos, it can aid in making the skin of the units more realistic.