Order in Chaos, the Answer...
Ok, before I get to anything else (and there will be a bit), I'd better answer the question I posed last month (BTW, notice that I've never let a full month go by without a blog entry -- I'll get the hang of this thing yet....)
Anyway, the reason, as far as I can see, is that there's a limited amount of track space at the 33rd Street terminal, so as a northbound train is approaching, a southbound train leave, to make room for it. They usually met as the 23rd St Station.