Xert recalculates your signature after every activity and makes adjustments anytime a breakthrough event occurs. Your signature can also decline from lack of activity and BT events.
So yes, Xert tends to correct your fitness signature over time. 
A 10w change is not unusual especially as your fitness improves through the phases and your training load increases.
Examples: Let’s See Some Recent BT’s - General - Xert Community Forum (xertonline.com)
If you use the Advanced MPA tab to change PP for an activity you want to click Extract first to see what the new signature will be then click Save to use that change moving forward.
One simple way to validate your signature is compare your HIE to PP in this chart –
Reference:
Are there errors in my Fitness Signature? – Xert (baronbiosys.com)