Single exhaust exits on right side - why

Over the years I have observed a lot of multi-cylinder motorcycle with a single exhaust mounted the right side of the bike. However, bikes with high-mounted side exhausts (SS style) usually mount on the left.

I've owned two Norton's, both MKIIIs (one bought new, the other used to replace the first after being stolen), and with the wicked 'spike' extension to the side-stand it seems ready made for a left side single exhaust, but I never saw one in any aftermarket catalogs or shops.

I'm not looking to purchase one, just bought two new peashooters, but I'm curious, anybody know why? I wonder Is it something like getting on a horse on the left side or is there a real reason?

You usually turn left on a race track and need more ground clearance on the left side? Primary takes up a lot of room on the left side.
High pipes exit on the left under the bike as it's being leaned and slid on a dirt track turning left, TT pipes are under the bike for turning left and right or twin high one each side when ground clearance is an issue, like when jumping or going over things.
Just my assumption.

Will

It’s a quaint old British custom, it was partly easier to put the exhaust pipe on a single on that side, and (for maintenance of primary chain, magneto e.t.c.) use the exhaust pipe on that side for the sidecar boys and girls who ride on the CORRECT side of the road, hence the sidecar shod Dominators had Siamese pipes on right.

Norton had bikes in the late 1920s that had the exhausts on the left. Some with complicated pipe shapes to get them there. Went right side in the 1930s, not sure I've ever seen that explained anywhere.

Since the US, most of Europe (and other countries) ride on the other side of the road, you'd think they'd have everything switched over to t'other side if there was any logic or reason to it, but not so.... ?