I could be wrong here, but I think the first time Luna was depicted with an abacus was in the fanfic Progress (It’s a good comedy fic, you should check it out). She uses it in the first chapter while trying to use a microwave, and gets pretty attached to it over time. Due to her isolation on the moon she misses the invention of the calculator, so she chooses to use an abacus simply because it’s an ancient mathematical tool she’s more familiar with. Any fanart showing Luna with an abacus is likely a reference to the fic. As much as I wish the reason were as interesting as badsitrep’s answer, I think the reality is simpler than that. As for Lyra, I’ve got nothing.
On that note, what do you guys bring to the pony discussion table?
All I’ve got is a weird obsession with fan fiction. My subject of choice is physics, and there’s never any reason to bring it up when talking ponies.
How do I quote multiple people on this forum?
I just figured it out myself. While most functions on the forums can use Textile (links, images, bold text etc), blockquotes need to use HTML. It uses this format:
< blockquote > text you want to quote < /blockquote >
(You have to remove the spaces from the tags on either side of the text)