Thanks! That does explain why it seemed to work sometimes and not others. I will give it another try. For the record, It looks like my skimming skills failed me, because I now see where this is explicitly noted in the docs:
When computing columns, you're working with lists of elements, and taking advantage of the fact that primitives like<and+automatically "spread" to lists. When performing comparisons, be sure to use=rather than~! If you want to call your own functions- say, to average within a grouped column- write them to accept a list
It even calls out “where” in the next paragraph.