Because the cell system doesn't actually know. CDMA doesn't work that way.
Every simultaneous call uses the entire spectrum, with separate encoding.
Each call looks like noise to all the others, and every time you add a call to the sector, the noise floor rises for all the others. But the amount of noise that each call adds to the noise floor is not constant. It's a function of the location of the phone and thus how much power it needs to use to get its signal through to the cell, and it's also a function of the actual voice traffic it is carrying, which varies from moment to moment. This leaves 61 for traffic channels . In practice, however, no cell ever can support 61 calls simultaneously.