TM4C1294NCPDT: Setting the PWM Period - Unexpected Results

Target

Tiva TM4C1294NCPDT Launchpad (EK-TM4C1294XL)

Note - XTAL (Y1) is 25MHz

Problem

The code below is designed to generate a 25 kHz PWM signal (T=40us) but is observed with a period of 31.1us.

Code & Scope Capture

int main(void) {

	//Clock Init
	SysCtlClockFreqSet((SYSCTL_XTAL_25MHZ | SYSCTL_OSC_MAIN | SYSCTL_USE_OSC), 25000000);	/* 25MHz external XTAL (Y1)				*/


	//******************************************************************************************************************************//
	//	 											    GPIO MODULE INIT															//
	//******************************************************************************************************************************//
	SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOF);							/* enable GPIO Port F									*/

	GPIOPinConfigure(GPIO_PF2_M0PWM2);										/* set pin-muxes										*/
    GPIOPinConfigure(GPIO_PF3_M0PWM3);

    GPIOPinTypePWM(GPIO_PORTF_BASE, GPIO_PIN_2);							/* set gpio-modules to push-pull output					*/
    GPIOPinTypePWM(GPIO_PORTF_BASE, GPIO_PIN_3);


    //******************************************************************************************************************************//
    //	 											    PWM MODULE INIT																//
    //******************************************************************************************************************************//
	//Init PWM Module
    SysCtlPeripheralEnable(SYSCTL_PERIPH_PWM0);								/* Enable the PWM0 peripheral							*/

    while(!SysCtlPeripheralReady(SYSCTL_PERIPH_PWM0));						/* Wait for the PWM0 module to be ready					*/

    PWMGenConfigure(PWM0_BASE, PWM_GEN_1, 									/* Configure the PWM generator for count down with 		*/
    				PWM_GEN_MODE_DOWN | PWM_GEN_MODE_NO_SYNC);				/* with immediate updates to the parameters 			*/

	//Init PWM0 Gen1
    PWMGenPeriodSet(PWM0_BASE,  PWM_GEN_1, 500);							/* 100% (40us) Tpwm_clk=80ns, 80n*500 = 40us (25kHz)	*/
    PWMPulseWidthSet(PWM0_BASE, PWM_OUT_2, 250);							/* 50%  (20us)											*/
    PWMPulseWidthSet(PWM0_BASE, PWM_OUT_3, 125);							/* 25%  (10us)											*/

	//Start PWM Module
    PWMGenEnable(PWM0_BASE, PWM_GEN_1);										/* Start the timers in Generator Module 1				*/

    PWMOutputState(PWM0_BASE, (PWM_OUT_2_BIT | PWM_OUT_3_BIT), true);		/* Enable the outputs									*/


    //spin
	for(;;) {
		_nop();
	}
}

See below for measured results, Ch1(Red): PWM3, Ch2(Yellow): PWM2 -

Question

Why do I see a period of 31us, not 40us?

Calculations Used

Fclk = 25MHz, Fclk_pwm = 12.5 MHz

Tclk_pwm = 80ns

N_per_pwm = 500

T_per_pwm = 40us (but 31.1us observed?)