Because each brand is manufactured slightly differently. Not all plates use the same composition, not all plates are the same thickness, each AGM has a slightly different vent pressure, each GEL has a slightly different vent pressure, not all batteries use the same % of acid and other additives in the electrolyte, some batteries are deep-cycle, some are starting and some are dual purpose. For example, the Firefly AGM is really not a true "AGM" and the electrolyte is really a hybrid between GEL and AGM. This is why it cannot be charged in excess of 14.4V. Some manufacturers give an acceptable voltage range. For marine use follow the manufacturers voltage guidance. This is why buying a charger that can be custom programmed and that has on battery temp compensation is so important.
Over the years many manufacturers have woken up and realized that 14.4V (where most of the FLA industry used to be) was murdering batteries used in a PSoC application (Sandia National Labs discovered this in the late 90's).. Many have now upped the absorption target to 14.7V +. Trojan is pretty precise at 14.82V and Rolls was a stalwart hold out until just a couple of years ago. Rolls too now advise 14.7V ++ for a PSoC application.
You can always charge your batteries how you wish, it's your money, but you won't be getting your money's worth out of the bank if it suggests 14.8V and you're charging them at 14.5V.... If the bank suggests 14.4V and they are VRLA batteries you can prematurely dry them out by continually over-charging them.