I noticed that the timing of a trace and and the sum of the nested steps doesn't always match. Is there a simple explanation for this?

Ops breakdown counts only the children of the transaction, and not the transaction's self-time. If we look at the trace, the first child span could start a bit after the transaction start, and the last child span could end before the transaction finish. There might be more non-span time in the in between as well. This will be considered the transaction's self-time. The rest will be considered the total time of the transaction's children, and will be broken down into operations. The transaction's self-time will be summed up with the child span time to make up the total duration.