I was looking at the Juno space probe which was launched in August 2011 onboard an Atlas V 551. In particular I was looking at https://elvperf.ksc.nasa.gov/Pages/News.aspx and using the performance query option to look at the maximum payload the Atlas V 551 could launch onto the orbit that Juno was launched on, which had a C3 of about 31.1 km^2/s^2. According to the website the Atlas V could only get 3270 kg's into such an orbit. This is at odds with the 3625 kg's that Juno weighed at launch.
Can anybody help me understand why there's this difference between the two numbers? I'm having trouble figuring it out.
Edit: http://spaceflight101.com/juno/juno-mission-trajectory-design/ would be where I got the 31.1 C3 number from. I also looked at the launch date (August 5th 2011), and looked at the required C3 by launch date from the final graph on this article https://trs.jpl.nasa.gov/bitstream/handle/2014/45416/08-2728_A1b.pdf?sequence=1
Edit2: The only thing I can think of currently is that it has something to do with launch 'declinations'. The NASA Launch vehicle performance calculator states that the rated performance is applicable for launch declinations between -28.5 and 28.5 degrees. From what I understand the further the launch declination goes from 0 for an orbit with a fixed C3 the less payload you can inject into such an orbit. Therefore if the numbers on the website apply for declinations up to 28.5 degrees it follows that declinations around 0 will result in higher payloads than stated on the website.
Does it sound like I'm on the right path here?