I have been building a large number of boards piles on Popham lately and I had this happen a couple times.
1) I start a timber pile with 10 stone. The last instance, I built about 7 at once.
2) Piles were built off claim
3) I few days have gone by since I started. I finished once last night, and looking back I believe it happened then too (BUILD progress went to 120%)
4) I walked up to piles and they were empty 40/0/0 and 10/0/0 but there was a sign post. If I put nothing in and walked away, the sign disappeared, but at one time it had 10 stone BUILT in.
5) When I finished the piles they built to 120% as if the previous stone was counted.
I thought this happened on other occasions, but I thought I was imaging it. Since it was multiple posts at once I could notice it happening.