I'll try a simplified example and see if it helps (fingers crossed!)...
Imagine there's a payment of 121 to be paid in 2 years' time. Let's ignore mortality, expenses, .... ( I like my numbers easy!) and consider only discounting. Let's assume a discount rate of 10%.
At time 0, if we have Liability = 121 discounted for 2 years at 10% = 100.
Suppose time 0 Assets = 100, so the starting surplus is 0.
When we get to time 1, if actual investment return in the year had been 10%, then assets would be 110. And if we didn't change the assumption, the liability (now discounting for just 1 year) would also be 110. So, no surplus arising between time 0 and time 1.
So, let's consider instead the situation at time 1 if actual investment return in the year had been 12% and if we changed the discount rate assumption to be 7%. So, time 1 assets of 112 and liabilities of 121/1.07 = 113. Surplus arising of -1.
When doing the analysis, we want to break this down as an experience variance (difference between actual and expected experience during the year) item of +2 and a 'change in assumption' item of -3. So, we want only want to change assumptions from the new valuation date and to compare the experience during the past year with the assumptions we had been making for that year.
Not sure how much any of that helps, but hopefully at least a little!