Colour Calibrating MacBook Pro Retina (Gamma 2.2 or 2.4?) with Spyder5

I received a Spyder5 and attempting to calibrate my MacBook Pro Retina's display.

I went through the process - initially got a colour cast, then after reducing my displays brightness to 120cd/m2 got a good improved calibration with the standard settings:

Gamma 2.2
6500'K
120 cd/m2

I then went thru one of the Display Analysis 'Tone Response' tests, and the measured gamma came back as 2.4.

I re-did the calibration using gamma 2.4 and it resulted in a colour cast.

My question is why the Tone Response test came back with a gamma 2.4 for my MacBook Pro Retina display?

My understanding is sRGB uses gamma 2.2. I haven't printed at the lab yet so not sure if its correct or not.

Nothing matters except print comparison. You have to get those prints.

Thanks. Will do that!

Make sure you follow my calibration instructions here. Do not attempt to follow the Spyder's own instructions, they're woeful.

Thanks Danien for the great info.
I just bought a new Spyder 5 PRO for my new iMac 27" 2TB.
There a few differences compared to my old Spyder 4 PRO with my old 24" iMac from 2009
Will do another full calibration tonight with lights out.