There can be two different scenarios that take place:
Scenario A: At the person level, in the Company info tab we see a company that they do not recognize and that is different than what is in SFDC, but there are no data value changes in the Activity log showing the Company name changing to that value
Scenario B: In the activity log we can see
- Change data value from <RandomValueA> to <Company name>
- Change data value from <RandomValueB> to <Company name>
But we cannot see any Change data value from <Company name> to <RandomValueA>
The issue is caused by the fact that one of the Contacts belonging to that Company had their company name changed within Marketo (for example via a Web Form Fillout or List Import). This change then propagates to all Contacts in that Company, but we do not log a change data value for this on the other records. We do log a change data value for the original Contact that caused the name change. Then, when Marketo attempts to sync the value to Salesforce, the company name from Salesforce overwrites all changes in Marketo.
In both Marketo and Salesforce you have a Contact object and you have an Account (Company) object. One Account can have multiple contacts associated to it. The Company name field lives on the Account object within Salesforce and not on the Contact object for both applications. In Salesforce (let's ignore Marketo for a second), if you change the Company name on an Account object, you will notice that all contacts under that account will belong to the new Company name. If you need to change the company for one contact only (not for all contacts), what you do in SFDC is create a new Account record with the new company name (if you don't have it already) and then move that single contact to the new Account record.
Now, in Marketo, you do not have the option of moving a single Contact to a new Account. Therefore, every time the Company name changes in Marketo, it will be just that - a change to the Company Name field on that existing Account - it won't mean that your Contact moved to another Account. It means that the existing (same) Account now has a new Company Came and, similar to Salesforce, that new Company Name value change will propagate to all the Contacts records under that Account.