According to Palmer's popular list of typical mashing and steeping yields, pale crystal malt (25-40L) has a typical steeping yield of 22 PPG. Slightly darker crystal malt (medium, 60-75L) weighs in at 18 PPG when steeped, and dark crystal (120L) at 16 PPG. Fine, that makes sense: darker crystal malts have lower steeping yields, consistent with how they behave when mashed.
But then we get to roasted barley and black patent malt, which both have a much lower yield when mashed, but are both listed as having typical steeping yields of 21 PPG.
What causes this apparent anomaly? I.e. why do these very dark malts have a lower mash yield but a higher steeping yield than crystal malts?