hmm. I'd speak with Support or check developers.marketo.com. The two cookies operate differently I imagine.
Let's say in reality,you have 100,000 unique visits.
GA sees 98,000 because 2% have javascript or cookies blocked.
Marketo sees 90,000 because 2% have things blocked, and 5% are inactive, and 3% are known so they don't show up in Anonymous.
So while those #s are totally made up, I could see that Anonymous has nothing to do with Visitors. If you did a smart list or report and looked for ALL visits in Marketo (Anony+Known) you should get closer to GA's #.