why is T of tropshpere at the equator -75oC?

[RobinEd]

I understand why and that the troposphere is higher at the equator than at the poles, and I also understand that it would be colder there, but I thought that after 36000ft / 11 km T stopped getting colder at -56.5oC. So why is the temp at the equator given as -75oC?
Cheers!