JohnL I understand what you say & have no issue with your "on average" analysis. An issue that must be understood that the steeper the incline the greater the risk of having a safety issue. Like I stated before a 1/10 (10%) is a steep grade a 1 in 4 (25%) is very steep (like mountain climping).
I realize operators are taught to negotiate a ramp with a load to travel w/forks up hill. ( I have developed & presented operator training prgrams during my 41 year career). In "the real" world I have personally witnessed many operators just believe this this as just a mere suggestions (kinda' like the Max. Speed Limit signs along the roadside) and travel in the opposite direction with a load. With steeper grades the greater the chance of a "ya ha" should the operator brake on the ramp as the load can be only tilsted back 8 to 10 degrees.
I understand ther are many situations that ramps are much steeper than 10%, in the States. "Most" of these situations arise in the older cities & buildings - like around NYC, Chicago, etc