We have a similar situation.
2 salesforce users send an email from Outlook add-in pressing 'Track & Send'.
User 1 - sent email to a lead, 'was sent sales email' activity appears, it is assigned to our designated Marketo account
User 2 - sent email to the same lead as User 1, 'was sent sales email' activity appears, it is assigned to User 2.
The process was exactly the same. They have the same permissions and profile type in SFDC.
Any idea what's causing this and how to fix it so the activity is always assigned to the person who sent the email?