Some fittings are undersized internally, which restricts flow; some are dimensionally inaccurate, which allows leakage.
Seems to follow price-point, in that I've had some problems with Russell fittings and no problems with Earl's. Speedway's fittings have been pretty good, though I've had mixed results with their tube-to-AN compression stuff.

When it comes to flammable liquids or lubricant supply...that stuff's pretty important, and leaks or failure can be catastrophic.