The LX has one 1.0 amp USB audio interface, while the EX/Touring have two 2.5 amp USB ports. However, the regulator inside your phone/device only will allow so much power at a time to prevent battery damage.
To take advantage of full/fast charging, the device needs to support it, and may need a special "fast charger" with the supported device. The rate of charging also depends on the device's battery level, with charge tapering off when battery is ~half full to prevent over-charging.
@dave92029 previously mentioned the Samsung S8 is rated at 1.67 amps, despite the 2.5 amp rating at both EX USB ports. For comparison, is your phone's amp rating, what was used to measure the 1.5 amp rate of charge, and what battery level was the phone/device at during that time?
I have the Touring trim. Here are screenshots showing the charging speeds for both the USB port (1.415A) and the 12V power outlet using a Samsung Adaptive Fast Charger (2.153A). I am using the same cable for both tests.
Easiest explanation is the port doesn't push power, the phone draws it.
Now breaking down the port ratings, a 2.5amp limit port means that above 2.5 amps some safety function will limit power, or the port will fail. There are other things to consider, if the port was warm for example it's at a reduced efficiency. The OEM ports could use less efficient components or lastly Samsung could use some sort of software/hardware function to limit draw on non-Samsung certified charging supplies (most likely in the adaptive charger plug). Similar to MFi, or other standards.