Hey Val,
When Marketo creates a lead in SFDC, it makes an API call that includes the values of all of the fields on the lead in Marketo.
It would be very confusing if this was not the case. For example, say that there is a checkbox field called "Opt In". The default value of this field is false as only want to send emails to leads who have explicitly opted in. A new lead then fills out a Marketo form and opts-in. Now, we have a lead in Marketo that does not exist in SFDC yet with Opt In set to true.
Now, we want to sync this lead up to SFDC. However, when we do so, Opt In turns back to false even though the lead already said in Marketo that they wanted emails.
As you can see, this behavior would cause unwanted scenarios. Thus, when Marketo creates a lead in SFDC, it pushes up all the values on the Marketo record instead of using the SFDC default.
As a workaround, I would create a trigger campaign in Marketo (using the Lead is Created) trigger, that automatically changes this checkbox to True when the lead is created. Thus, you will essentially have a default value of True in both Marketo and SFDC.
Best,
Will