Xert is showing my 5 minute power as 395w and ftp 310w. The ftp is quite accurate from what I know, but I’m sure I’ve never held 395w for 5 minutes. Wattsboard is showing my highest 5 minute power as 356w.
Is there any way to see where it’s grabbing the 395 number from?
Xert looks to establish your response to work above threshold and how this makes you fatigue. This is MPA. Based on this, it quantifies what it thinks you can do for 5 minutes. (5 minutes efforts are really hard ones to express your full fitness with but many athletes can.) Surprise yourself with a test. 
I might do that! That was me on the Facebook post questioning 5 minute power. Makes sense now. I’ll give it a go when I’m feeling relatively fresh.