Here are some considerations that might help explain the discrepancies in implied volatility (IV) calculations for ultra-short options, like 0DTE (Zero Days to Expiration) SPX options, between different brokers such as Interactive Brokers (IB) and Charles Schwab:

1. IV Calculation Techniques: Different platforms may use various methods to calculate IV. Some might rely on the Black-Scholes model, while others could use binomial trees or proprietary algorithms. This can lead to different values for implied volatility.

2. Put-Call Parity and Expiration Effects: As options approach expiration, the principles of put-call parity apply differently, particularly for American-style options. This can result in variations in IV as expiry approaches.

3. Market Liquidity and Order Flow: The immediate demand and supply dynamics can lead to substantial fluctuations in IV. Options nearing expiration are particularly susceptible to these market forces, which could lead to differences in the IV reported by different brokers.

4. Risk-Free Rate and Dividends Assumptions: Different assumptions about the risk-free rate and expected dividends can influence the IV calculation and vary across different trading platforms.

5. Volatility Surface Adjustments: Some platforms make adjustments to the volatility surface to account for market conditions, which can lead to differences in short-term IV calculations.

6. Sensitivity to Market Inputs: Ultra-short options, such as 0DTE options, are very sensitive to changes in the underlying asset's price, time decay, and volatility itself, amplifying any discrepancies in input assumptions.

Understanding these factors might help you manage differences when interpreting IV from different brokers. If these discrepancies are critical to your trading, it might be worthwhile to use a standardized method for your own calculations or consult platforms with models that align closely with your perspective.

Sources: Pricing - QuantConnect.com, Greeks and IV Implementation, Key Concepts - QuantConnect.com

Just before several minutes till expiration IV goes to almost 0 (0.03-0.07) when IB or vanilla BS gives much higher values (depending on the market, but it can be 0.6-0.8 last days)