I honestly don't know, I tried to have a look in my RCE but I'm waiting 4 minutes so far and it still hasn't finished loading the welcome screen. Have you looked at all the different ways that the field gets populated? i.e. Marketo Form, Salesforce, List Upload. Also, if its Marketo forms or SF, are the field types drop downs etc where someone might not select a choice.
If there has been a huge jump in the numbers it could also point to a list upload into Marketo or your CRM.
Thats my 2 cents worth
If your Account Type field isn't being populated properly, then it's likely an issue with SFDC. BUT, if this report looks at Leads mostly, then the Account Type would be NULL right?
Marketo RCE updates overnight and sees whatever you allow it to see. Are you sure this is the correct Account Type field?
I do not use SFDC, so it cannot be anything with that. The problem is that this (REC Report) normally reflects numbers very similar to pulling a smart list looking at these leads. The smart list when I run it looks right on par with normal monthly reports. I think that perhaps the numbers RCE is not pulling the numbers correctly. We don’t have any forms to speak of that would really affect this either.