esp32 pico kit - actual clock speed vs crystal vs max_speed etc - arduino

Question

I am trying to understand how the clock speed in the esp32 works.
I am running arduino ide 1.8.12 with 1.04 board definition - on a pico kit 4 module .
The device datasheet says it has a 40mhz crystal.
getCpuFrequencyMhz() reports 240mhz
But the following code is generating 1sec interrupts with 80mgz as a base i.e., as if the clock speed is 80mhz.
If I explicitly set the clock speed to 80 mhz then getCpufrequency correctly reports it as having changed, but the code still keeps generating 1sec interrupts. If I set the clock to 40mhz then I finally see interrupts at 2 secs as expected.
I am sure that the explanation is somewhere in expressif's docs, but I have been unable to glean it. Can anyone quickly explain what is going on or point me to some resource that explains this particular circumstance?
---------------
Sample output
getCpuFrequencyMhz() = 240
interrupts so far = 1 - : - mills() = 1018
interrupts so far = 2 - : - mills() = 2018
interrupts so far = 3 - : - mills() = 3018
interrupts so far = 4 - : - mills() = 4018
interrupts so far = 5 - : - mills() = 5018
----------------Sample code
#include "esp32-hal-cpu.h"
volatile int interruptCounter;
int totalInterruptCounter;
hw_timer_t * timer = NULL;
portMUX_TYPE timerMux = portMUX_INITIALIZER_UNLOCKED;
void IRAM_ATTR onTimer() {
portENTER_CRITICAL_ISR(&timerMux);
interruptCounter++;
portEXIT_CRITICAL_ISR(&timerMux);
}
void setup() {
Serial.begin(115200);
Serial.printf("getCpuFrequencyMhz() = %d\n",getCpuFrequencyMhz()); //Get CPU clock
timer = timerBegin(0, 80, true);
timerAttachInterrupt(timer, &onTimer, true);
timerAlarmWrite(timer, 1000000, true);
timerAlarmEnable(timer);
}
void loop() {
if (interruptCounter > 0) {
portENTER_CRITICAL(&timerMux);
interruptCounter--;
portEXIT_CRITICAL(&timerMux);
totalInterruptCounter++;
Serial.printf("interrupts so far = %d - : - mills() = %d \n", totalInterruptCounter, millis());
}
}