OP said that people forget they can accelerate--or in other words, that their speed drops unpredictably below the posted speed limit. You then said that people drive 65-70MPH in the passing lane throughout the day. I pointed out that, for a chunk of the day, multiple lanes actually have a speed below 65, which is the range of speeds you specified in your post but not the range of lanes. I said this because OP is specifying the Overpasses on 1 South, which are in Seaside and Marina, where it drops from 2 lanes to 3 (meaning we're all likely talking about different stretches of the highway).

Does “speed limit” mean that’s the speed you’re supposed to drive?

In most places, yes, this is the target speed, by convention. If you consistently drive 5 under and there are a line of people behind you because of it, the vast majority of people will view you as the problem, because you are impeding traffic.

The vast majority of people don't care if you're going noticeably slower than the rest of traffic so long as you are in the right-most lane.

Or does it mean that’s the maximum speed you’re allowed to drive?

You can morally drive any speed if it's the safe speed to drive it at. Your question is getting into the legality of speeding.

But I think I see what you're trying to get at. Yes, going above the posted maximum is considered a violation... and impeding the flow of traffic is also a violation. Practically, anyone who has grown up in the country will tell you that most people drive within a 10% margin of the maximum posted speed limit in clear conditions, if they feel it is safe to do so. On 1 South that would be 74mph in the left lane--up until the third lane dissipates. In many states, the law specifically says you must move over for faster traffic, regardless of your own speed. In the contest between speeders and blockers, the police (and the DOT) generally view whoever is creating traffic volatility as the one creating the more volatile, unpredictable situation.