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Created At is the date the initial lead record is created in Marketo, which includes anonymous lead records without email addresses, so you might want to use a different Group By field such as SFDC Created or a custom Lead was Created field that has the date the lead opted-in and provided their email address. Be aware though that tihs really doesn't show how many were mailable in that month - it only shows how many are currently mailable by the month in which they were created. You'd have to run a monthly report and preserve those values in a spreadsheet or a static list to have a historical record of this, but even then some leads may have been mailable for only a portion of the month, but this is more accurate than running the list today and assuming the leads were mailable in the month they were created..
The Not Email Bounced filter will exclude both permanent Category 2 Email Bounces that sets the Email Invalid field value to 'true' as well as temporary Category 1 Email Bounces that set the Marketing Suspended field value to 'true' for 24 hours. You may not want to exclude the Category 1 Email Bounces from your mailable list since they are temporary.
Your smart list does not exclude leads that have their Unsubscribed, Blacklisted and not necessarily all Email Invalid field values set to 'true'.
Your list does not include leads with Soft Bounces that have the Detail field value 'System send failure: To address parse error…' or 'System send failure: Email injection error… '. These leads are actually not sent emails by Marketo because their email addresses have invalid formats or characters.