It is probably over-thinking. Most of these 'short term' cash ETFs have very similar maturities or interest rate exposure and are essentially nearly-identical (except as noted below). Using 'indicated' yield as a comparison can be misleading as it will change. That being said, there are differences between T-bills and money market securites. T-bills are government obligations, money market are corporate obligations. Thus, money market funds have a higher degree of risk and should pay more. But considering the very short term nature of obligations, we would consider this additional risk quite low---just not 0.
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