The LR4 has a battery setting that needs to be changed to AGM since it requires 0.3 or more Volts for charging. The LR3 does not have the setting so your essentially stuck with the lead acids batteries unless you want them to die prematurely. It's just chemistry an poor charging design by Landrover.
"Never designed for AGM" - that is an interesting thought.
It is one I never thought about but yes, per the jpg, my 2005 factory battery was one of those low maintenance lead acid batteries with the sort of removable flush caps, but not the older style screw caps.
I always wondered why the LR4 has a Car Configuration File (CCF) switch for battery type - never thought about the increased charge voltage that the AGM wish to see.
I also know that there was a change in "smart regulator" design re the 3's but never really understood what that was about. In saying that, my information is that the battery charge voltage is set by the car computer software and not the alternator regulator as it is/was in lesser vehicles - that is all but the Land Rover.
The different charge voltage would be consistent with that switch in the LR4 CCF - the car computer software dictates battery charge voltage.
At the same time, I note that in colder weather the system voltage is closer to 14.5 VDC measured at the OBD plug so that seems adequate for the AGM charging. At the same time, 13.3 VDC is a number I see in hot weather and I suppose that is too low to charge the AGM..
Below is a link listing the various battery types and suggested charge and float voltages.
Maybe I should stop buying their Interstate AGM and go back to the older style low maintenance battery - an interesting idea as the way the AGM fail does not impress me - kind of instantly; I much preferred the "Gee, the battery is getting weak - should change it before winter," kind of thinking.
https://www.batterystuff.com/kb/articles/5-battery-types-explained-sealed-agm-gel.html