When the device boots up, the SCLK_LF source is derived from RCOSC_HF and the derived clock frequency is around 31.250kHz.
During start up, in SetupTrimDevice (driverlib function), it configures the SCLK_LF source and the subsecinc value(RTC register) based on CCFG option. At this point we are programming RTC based on the requested SCLK_LF source, even though the current SCLK_LF source is not the requested one.
XOSC_LF runs on 32.768kHz and subsecinc is programmed to account for that. This means until XOSCLF is ready and CC26x2 switches to it, we are using the subsecinc value that does not reflect the actual sclk_lf frequency
The longer time it takes to switches to XOSC_LF, the larger the error is.