Sierra Squared, an ejumicated guess.
The F/O, who I presume was the PNF, had less on his/her hands and was able to observe the IAS either lower than expected for the phase of flight and power set, and/or decaying because of ice accumulation, and/or decaying because of wind shear.
Any of the above would be cause for the PNF to bring to the PF's attention, the fact or suspicion that all was not as it should have been with regard to the IAS.