Watch running fast despite broken mainspring?

I just picked up this beauty from having it serviced. So glad to have it back, and it’s running smoothly at -1 sec/day.

Before service, all functions (time, date, jumping hour hand, GMT) worked fine but the watch ran circa 2 mins/day fast. The repair specialist told me that they needed to replace a broken mainspring. Huh, I thought. I did not know that a watch could fully run, be wound, maintain a decent power reserve, and also run fast, despite having a broken mainspring. Can someone explain this to me? I’m super curious.

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Thanks, that's also just a helpful bit of knowledge in general.

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incredibly well-done! You should make a separate post about that website; I think a lot of folks here will appreciate it and learn something.

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Great looking watch. I never think Omega when I think GMT but that one might change my mind.

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Looks fine to my eye but not my watchmakers...He was right it runs so much better!~

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