Remote stopped working (part of Condo gate) - Others continue to work

Opener is Chamberlain Elite SL3000UL for gate operation
Receiver (recently replaced) is Multi-Code by Linear model 1099-50

We are part of a condo association with a gated parking area.  We were giving one remote for our unit.  It was very old, didn't stay together anymore...and finally quit working.  Unfortunately, I don't know the brand of remote.

We bought one off Amazon (Linear Multicode 3089 Compatible Visor Remote Opener), and it worked great...so we bought another.

They worked for about 6 months, we left for 3 weeks vacation, and when we came back, they don't work anymore.  Already changed the batteries, and still nothing.

We asked the association if something changed, they said they did have to change something (he didn't know exactly, but thought it was the receiver), but the code was the same.  They suggested we call the door company, and they want $100 just to show up.  I'm retired and have large medical expenses, so $ is tight.

This morning, I opened up the opener and found the receiver.  The pins were set to 310 Mhz (non Stanly), and it appears our remotes are at 300mHz.  I changed the pins and my remote works, but no one else's does. I changed it back....

I'm buying a Remote at 310MHz to see if it will work.

Why would everyone's remote work, when some had 310 and some had 300MHz before?  I understand now why the 300 doesn't work, but why would both have worked before?