GMT II time-keeping accurate only when fully wound

[stanima]

I thought my GMT II (16713) was keeping lousy time. I purchased a winder, and since I got it I have been keeping it on the winder when it isn't worn.

So before I got the winder it was losing 3-20 seconds a day. On average a minute every 2 weeks. My normal wear period was 8 hours a day, as I work from home and don't put my watch on until I headed out. That was usually between 2 and 3 pm, and then keep the watch on until bedtime, 11 or midnight.

Since I have been using the winder, exactly one month ago, the watch is exactly 1 second slow. Or 0.032 seconds per day.

My question- is that normal? If the watch is only partially wound will it run slower? I would have thought that with a good mainspring the watch would run the same whether the power reserve was full or nearly depleted.

Watch was fully serviced at the beginning of 2016 by Rick Dietel.

Thanks.