I'm not sure why that would be different between XIM versions. It's possible the two XIMs are using different config settings or one is using older support. Existing config's don't receive updated support automatically so this does happen.
One thing you could do is set APEX to 125hz, save, and then reconnect it to the console. Use default sync and APEX will run as a XIM4.
Is this happening in the PC version or the Xbox version?