According to Wikipedia the two variants exist because the digits of large numbers used to be grouped into groups of six digits but in order to improve readability this was eventually changed to groups of three digits and some insisted that with that also the naming should be adjusted. A long scale trillion has three digit groups when using groups of six digits (1,000000,000000,000000) and six after the switch to groups of three (1,000,000,000,000,000,000) which then should be a short scale sextillion but somehow it ended up as a quintillion.