Yes, possible. Although I think your 10 degree difference is quite a bit.
Look at it this way, more power, less throttle input.
Less throttle input, lower EGT
, lower turbo temperature.
Also quite possible fueling is changed at that RPM band that gets you 75mph, when comparing stock to tune.
Oil cools turbo, sheds heat to cooling system. Less heat to deal with, lower EOT
What is the coolant temp, and EGT
with the performance tune?
Your tuner should be able to provide insight......