Those with long memories will recall that the USA at one time had three levels of airways, low...to 14,000 feet, intermediate 14,000 to 24,000 feet, and jet airways, FL240 and above.
When the change was made for jet airways to be lowered to 18,000 feet, and at the same time (or slightly later) the positive control area (class A airspace) was to be lowered to FL180, both east and west transistion levels were set at...18,000 ft/FL180.