A few thoughts here, I think it depends on your strategy, really and the reasoning for why you deleted them out of SFDC to begin with. A few of my clients just add boolean data of 'do not sync to Marketo' between SFDC and MKTO and usually just purge from both platforms. However, if there is a chance for re-engagement in the future, than keep these leads in and they could be a new marketing campaign later on. Also, keep in mind that keeping you database clean and lowest count as possible is better for your pricing package. I think you need to ask the question of 'why' would you want to keep the leads in MKTO and think of why they were deleted in SFDC.
Let me know if that doesn't help and we can chat further.
I don't know the exact reason from all time why the leads got deleted but from last one year, I am noticing that sometimes reps delete the leads if they find duplicates or we are using ring lead to clean up the lead data.
The number 31622 seems to be very high so I am analyzing the benefits of keeping these leads in Marketo.
I am thinking of another strategy where I will keep only unsubscribe leads so that we don't add/ purchase the same email or email the unsubscribed.
I know every company has a different strategy that's the reason I want suggestions.