MSP430FR2311: RTC Interval Incorrect

Please refer to example code:

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//******************************************************************************
//  MSP430FR231x Demo - RTC, toggle P1.0 every 1s
//
//  Description: Configure ACLK to use REFO as RTC source clock,
//               ACLK = REFO = 32kHz, MCLK = SMCLK = default DCODIV = ~1MHz.
//
//           MSP430FR2311
//         ---------------
//     /|\|               |
//      | |               |
//      --|RST            |
//        |               |
//        |               |
//        |          P1.0 |---> LED
//        |               |
//
//   Darren Lu
//   Texas Instruments Inc.
//   Oct. 2015
//   Built with IAR Embedded Workbench v6.30 & Code Composer Studio v6.1
//******************************************************************************
#include <msp430.h>

int main(void)
{
    WDTCTL = WDTPW | WDTHOLD;               // Stop watchdog timer

    P1OUT &= ~BIT0;                         // Clear P1.0 output latch for a defined power-on state
    P1DIR |= BIT0;                          // Set P1.0 to output direction

    PM5CTL0 &= ~LOCKLPM5;                   // Disable the GPIO power-on default high-impedance mode
                                            // to activate previously configured port settings


    // RTC count re-load compare value at 32.
    // 1024/32768 * 32 = 1 sec.
    RTCMOD = 32-1;
                                            // Initialize RTC
    // Source = ACLK = REFO, divided by 1024
    SYSCFG2 |= RTCCKSEL;                    // Select ACLK as RTC clock
    RTCCTL = RTCSS_1 | RTCSR | RTCPS__1024 | RTCIE;

    __bis_SR_register(LPM3_bits | GIE);     // Enter LPM3, enable interrupt
}

// RTC interrupt service routine
#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector=RTC_VECTOR
__interrupt void RTC_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(RTC_VECTOR))) RTC_ISR (void)
#else
#error Compiler not supported!
#endif
{
    switch(__even_in_range(RTCIV,RTCIV_RTCIF))
    {
        case  RTCIV_NONE:   break;          // No interrupt
        case  RTCIV_RTCIF:                  // RTC Overflow
            P1OUT ^= BIT0;
            break;
        default: break;
    }
}