Drop in SPY ETF Constituent

Hi, 

Any reason why the number of sp500 constituents drop over time maybe the way i'm calling it's not good.

Thanks for your help 

class StandardizedUnexpectedEarnings(QCAlgorithm):
 
    def Initialize(self):
        
        self.SetStartDate(2020, 3, 1)                               # Set Start Date. Warm up for first 36 months
        self.SetEndDate(2024, 6, 6)                                 # Set End Date
        self.SetCash(100000)                                        # Set Strategy Cash

        self.add_universe_selection(
            ETFConstituentsUniverseSelectionModel("SPY", universe_filter_func=self.etf_constituents_filter)
        )
        
    def etf_constituents_filter(self, constituents: List[ETFConstituentUniverse]) -> List[Symbol]:
    
        if self.time.month == self.month:
            return Universe.Unchanged
        self.month = self.time.month
        self.debug(self.time)
        self.debug(len(list(constituents)))
        return  Universe.Unchanged

That my logs output :

2020-03-01 00:00:00 504
2020-04-01 00:00:00 503
2020-05-01 00:00:00 504
2020-07-01 00:00:00 505
2020-08-01 00:00:00 504
2020-11-03 00:00:00 503
2021-04-01 00:00:00 504
2021-05-01 00:00:00 503
2021-07-01 00:00:00 502
2021-08-03 00:00:00 501
2022-03-01 00:00:00 500
2022-04-01 00:00:00 1000
2022-05-03 00:00:00 498
2022-11-01 00:00:00 496
2022-12-01 00:00:00 497
2023-01-04 00:00:00 496
2023-02-01 00:00:00 497
2023-03-01 00:00:00 496
2023-04-01 00:00:00 497
2023-06-01 00:00:00 498
2023-07-01 00:00:00 497
2023-09-01 00:00:00 496
2023-11-01 00:00:00 495
2024-04-02 00:00:00 496
2024-05-01 00:00:00 495