(1) You were at altitude, and therefore CAS<TAS. The aircraft stalls at a fixed CAS, not at a fixed TAS.
(2) GPS lag - it's not an instantaneous speed device and needs to stabilise for some seconds - something the aeroplane is unlikely to do.
(3) ASIs are calibrated down to 1.3Vs and up to Vne, it's common to get greater than the permitted errors outside that range.
(4) It's common at high AoA for the ASI to significantly underread due to poor flow into the pitot.
(5) You were lighter than MTOW, which is the weight at which Vs is quoted.
(6) GPS only gives groundspeed. Descending at a high RoD you are actually flying rather faster than GPS will give.
(7) You are mixing up knots and mph somewhere along the line.
(#) Any combination of the above.
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