Bad answer: It's a fault in our system. Something we've never fixed. When you're merging two profiles you can select which data to keep from which profile. But with marriage date and location, it just keeps the data from the merged-into profile. You're not given the choice and it doesn't show you what it's doing. (The reason we don't offer this is that it technically it involves a few extra steps on the back-end. Marriage data is stored separately from individual person data.)
Until we fix this, I'll add an explanation to the merge page.