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When a field value on the Company/Account object is changed, all lead records associated to the same Company/Account inside of Marketo will have that change made to them as well. The problem we find is that the Change Data Value activity is only logged on the individual lead record where the change was actually made. All other leads associated to the same Company/Account that got the change due to that association will not show the Data Value Change activity at all.
Since SFDC is the owner of the Account object, the next time it syncs, the value from the SFDC side will overwrite the other value. That's why you don't see the value change to the wrong value but do see the correct value coming down from SFDC.