difference between modules

I have two modules (XB8-DMRS-002 rev G) on different PCB.
pic. 1 - https://ibb.co/HnwvS2F
pic. 2 - https://ibb.co/3CYGJkc

first, on (pic. 1) is working fine.
but one of them, on (pic. 2) did not work with default U. FL connector.
I understand my fault too late. so i used solder to set up my antenna at RF pad. And now it is working.
question is why first xbee (pic. 1) still working with U. FL connector?