The 5xxx series, which includes 5160 and 52100 have chromium in them. Chrome is notorious as a carbide former. But, the excess carbon/carbides will dissolve if you're at the right temperature and anticipate soaking a little longer than might be normal for say a 10XX steel. That way you can get maximum performance out of a tricksy steel because you're quenching from maximum carbon input rather than partial.

Those pesky little minor alloying elements don't have to be present in large quantities to make big effects. Steel is a good teacher. Sometimes good teachers are pains in the butt-ocks.