Perfect. So… Your ATL flight lands at 07:45, and your SFO flight departs at 09:00. The minimum connection time is 01:15. All looks good, however you have to remember we are dealing with computers, who work on exact values, but then simplify for us humans.
So what about seconds? Assume that you SFO departs at 09:00:00, with a minimum connection time of 01:15:00, that means your ATL flight would have to arrive at latest at 07:45:00. What is not displayed is seconds, if your ATL flight arrives at 07:45:01, then it is no longer a valid connection as it’s 01:14:59 connection, but 07:45:01 would still be displayed as 07:45.
A good technique is to add a minute to the minimum connection time to avoid this, so try scheduling an arrival at 07:44 instead.