As Cheddar was trying to say, the transactions that are "transfers" may show up under your total in BoA but mint disregards them under your total spending, because mint is looking at your entire financial picture. So it realizes that if you pay off a credit card, it already captured that as spending on with your credit card and no reason to capture the CC payment as spending again.
The same with investment buys, you are just transferring money to another account, but actually spending the money.
Where as your BoA account considers that money spent when you pay american express or buy vanguard mutual fund shares ect, so it will look like you spent more when you just review your BoA statements.
If you export all transaction in mint for those accounts to excel, the total should be equal, because it will show the payments and investment buys, just doesn't show up in your trends.
I would also advise, as a quick easy check, export both BoA and Mint transactions to excel, and sort transactions by highest value on each then compare all of your big expenses, until you are confident that everything looks right