Regional differences are correct. EU (and a lot of the rest of other places too) used 2 parallel lines to indicate a non-polarized capacitors. In the USA and nearby places it is common to use the line and arc even for non-polarized types... making the only difference between polarized types and the non-polarized types the "+" symbol.
My feeling is that the arc side of the symbol has its roots from the days of very early radio schematics and normally indicated, for convenience only, the side of the capacitor that was closer to GND potential and not a particular characteristic of the part itself. Japan even uses yet another symbol for capacitors.
Old conventions take a long time to change and the influences like Popular Electronics, American Radio Relay League and other American style publications on the American "old farts", still persists today. Elektor, Everyday Practical Electronics and Silicon Chip, for example are publications that perpetuate the more understandable "---||---" parallel plates symbols and are more sensible because there really is no need to show polarity on a non-polarized capacitor. The "+" symbol is really 'good enough'.
In really olden days, capacitors also came in big metal cylindrical cases and it was believed that a 1 FARAD capacitor (a value now seen in super caps) would need to be a big as a delivery truck. Some things change... some things stay the same.
For example, I never could get the hang of EIA-logic gate schematic symbols style versus JEDEC style. I learned JEDEC style and my brain would not let me change.