Why is that a simple allocation of 100% of portfolio to SPY shows that half of the buying trade uses margin?

For example, with a $100k portfolio and self.SetHoldings("SPY", 1), charting self.Portfolio.MarginRemaining and self.PortfolioMarginUsed shows that the portfolio value is indeed $100k on day 1, but $50k of margin was used to make the purchase. Since there is ample cash, I would expect no margin to be used. Why is half the margin available being used on this trade? Thanks for your help.

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