The free credit score from Mint.com is not a FICO score. Citi does give us Equifax FICO score but even with FICO there exist so many variations. Fair Isaac has developed around 50 different scores using different formulas - only very few are actually accessible to consumers. Most free credit scores are simply FAKO scores (which are never used by lenders). They can choose to put more weight on some element of your credit report; or ignore youyour history as an authorized user (which is a common credit rebuilding technique). It is just pointless to directly compare credit scores from different sources, because that tells you nothing other than that they are different.
Now there are still ways to utilize those free scores. You can try collect them periodically and watch the trend of each score. The idea is that, FAKO or FICO, the factors that affect your score are very similar. If you see your Mint.com score going up two months straight, chances are that the actual FICO scores that lenders use would also go up. I was able to verify this with my CreditKarma, CreditSesame and Transunion FICO scores.