On my last trip...
On my last trip between LAX and Anchorage on Alaska Airlines, I carried onboard a fully loaded Lowepro Vertex 300 backpack. That was within the size limitations but, they never weighed the bag. It was PRETTY DARN HEAVY and might have been above the restrictions....
It seems strange but, I was charged an extra $25 for my checked on bag because it was above the 50# weight restriction but, was not charged for it on my way home - and it was even heavier than.
Also, I asked the security guard at the luggage search if I could carry my monopod onboars or if I would have to check it. In LAX I was told that it was a "clublike" weapon and I couldn't carry it aboard. At Anchorage, I was told "Sure, people carry these onboard every day!"
I am not sure that there is total standardization of rules or enforcement of rules.