Theory based on further findings:

Stars rounds down each year when applying credit for partially completed items.
The rounding is down to an integer % complete.

For example, if you finish 999 resources of a 1000 resource item, stars gives you credit for 99%, not 99.9%. You'll have to spend 10 resources the next year, not 1.

In other words, for multiyear builds, you lose an average of 0.5% of the cost of the item for each year other than the year of completion.