It's probably measuring other parameters of your face. My passport photo is without specs, but I do not have to take them off to go through eGates.
The Thai system is comparing the gate camera with the photo which they have taken and keep on file. But even so, there is no way the resolution from their camera, or on my Passport photo can reveal that finer detail.
They have facial recognition abilities in a lot of countries, but none taking iris scan into consideration.