Seat lug misalignment?
 

I'm not a framebuilder--I hang out almost exclusively on the Classic and Vintage site. But I'm wondering if anyone here can give me any perspective on the frame of my late father's Raleigh Sports, which I have been riding off and on for the past few years.
Note that the seatpost has a "setback" from the seat tube (shows up better if you click on the photo to enlarge it). Contrary to what you might think, the post is perfectly straight, and about 3-4 inches of the post extend into the seat tube--presumably the ID if the tube is substantially larger than the OD of the seatpost (although the seatpost is the correct size and fits snugly at the lug itself).
More to the point, I remember the day in 1966 when my dad brought the new bike home from the bike shop--I think I was the first one to notice the seatpost issue. In other words, it's a manufacturing defect that's been there since the bike was new, not a matter of damage. Since it didn't and doesn't cause any problems, my dad just lived with it. So do I, although as you can see from the extra-long stem and seatpost the frame is a bit small for me. In fact, that being the cause, the offset post is actually beneficial, because it effectively makes the top tube longer.
But here's my question: does anyone know what the framebuilder at the Nottingham factory did wrong all those years ago to get that result? My guess has always been that it's a case of "tolerance stack-up:" maybe the sockets in the seat lug were a little oversized, while the tubing was a little undersized, and Nigel (or whatever his name was) assembled it with the lug rocked backward as far as it would go, then brazed it together like that.Attachment 163607 Is that a possibility? Any other ideas?
Thanks for any suggestions--not important in the least for my continued use of the bike, but I'm just wondering.

Is there a spacer at the top of the seat tube? I don't think I can really remember a tube that expands below the seat lug. There are all sorts of things that can go wrong in a factory, and the UK/European bike factory workers weren't known for their care in assembly. There may have been hammers involved.

Nope, no spacer. It beats me how the seatpost slips right into the tube--you'd think it would bind toward the front of the tube.

Could be many things, wrong lug would be my guess.

That's a great thought. Maybe a lug that was meant for a DL-1--the 3-speed with big 28 inch wheels? They had more relaxed frame angles, so that would seem to make sense.