Disclaimer: This is pure speculation, as I don't design laptops, and as you said there is no standard so it's hard to prove a negative.
This question on Quora about why charging ports are typically on the left inspired my answer: https://www.quora.com/Why-are-charging-ports-on-the-left-sides-of-all-laptops
Most people are right handed. They use mouse by placing it on the right side of the laptop. Since mouse only requires USB 1.0 /2.0 port, Laptop manufacturers keep these ports on the right side of the laptop.
So as it says a majority of people are right handed so they leave the usb ports on the right for mouse users. If you Google motherboards, you see that manufacturers seem to keep all of their I/O ports together (I don't know enough about hardware to say why, insert language about "serial bus" and "form factor" here) this includes the USB ports for the mouse as well as video and audio ports.
So I wager that it wasn't a direct decision to put the headphones on the right. They most likely made the decision to put the USB port on the right and the other ports just came with it.