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The slightly good news is that this was because both the employment ratio (shown inverted here) and the labor force participation rate both improved in May, cancelling each other out.
The good news stems from the fact that the labor force participation rate actually increased (very slightly). It has fallen dramatically over the past few years and – as the denominator in the U3 rate – has been the main driver for the fall in the umemployment rate. (Less people looking for work, means less unemployment.)