Smartphone Fast Charger with Lipo and UBEC
Shouldn't a 5v@3A UBEC be able to match or beat this speed?

Indeed the amount of power (15W) is enough but the phone determines if it will fast charge or not.

How does my phone know that one is the fast charger and the other isn't?

Indeed your phone does not recognize the UBEC as being capable of providing the power so the phone does not "know" it can draw 3 A at 5 V. Very likely it also cannot draw 3 A at 5 V, it will need 9 V for the fastest charging and converts that 9V into a lower voltage which also increases the current (using a switched power converter).

The phone and the official Samsung charger communicate through the data lines in the USB cable, the charger puts some voltages on those lines and the phone detects that. Obviously the UBEC charger doesn't do that so the phone plays it safe and assumes that it can only draw a modest current making charging slow.

If you used a USB cable to connect to the UBEC module, what you can do is short the two datalines, usually these are the green and white cables. Then maybe the phone sees the UBEC as a charger and it might charge faster. I have used this trick (shorting the datalines) many times before and it often helps to increase the charging current. It does depend on the phone if this will work though so no guarantees!