I can give an answer the second part of your edit. In my experience terminology, once established and active with a critical mass tends to propagate and then remain. It doesn't really change much. Particularly in trading, terminology has evolved around the central points of liquidity, and becomes embedded in many systems; computerised, vocal and written contracts. Often it forms based on the fastest way to express something when agreeing a trade. In the cross-currency swap market for example I speculate that the sole reason that the voice broker market quotes "euro-dollar" (in that order of currencies) as opposed to "dollar-yen", when they are technically the same similar product, is simply the nature that it rolls off a western tongue faster and easier than the converse. And in the dollar-yen case it avoids having to have notionals like "1.1 trillion yen please", cause you default to USD.
As a concrete example, some terminology that appeared in the EUR IRS interdealer market 2y ago is the name of the price "10Y gadget", which is the yield differential between 10Y bond futures and 10Y swaps. I found this an awful name and lobbied to change it to "10Y spread", which is common in GBP and USD. But when you find there is no real (simultaneous) will to change it doesn't get much traction and as long as everyone uses it and understands it you just have to go with it.
Perhaps there is no legitimate explanation for the appearance of the '/', and after battling the initial confusion no one cares anymore.