Speed limits are trickier than structural safety margins because of several factors:

• In some areas, particularly remote areas, the process isn't very well defined. Sometimes the speed limit will be set by one guy who just felt like that was fine. Doesn't even have to be an engineer really.
• Standards evolve over time (trending towards lower speed limits) but speed limits only change when a tragedy or major road renovation happens. Where I live there's sometimes a 40 km/h spread on posted speed limits for similar roads depending on whether they were rebuilt last year or 50 years ago.
• Car culture means drivers hold a ton of political power. There are a myriad of traffic devices that cannot be built not because of practical or financial constraints, only because they would "inconvenience drivers". Lower speed limits are often one of those. People complain so the government backs down despite engineering recommendations.
• A driver is always liable if they drive too fast for the conditions, not the traffic engineer. That goes to the previous point, with zero penalty for not sticking to the sensible engineering choice, political pressure easily wins out. Hard to argue against a work order when the person signing off on it cannot be sued for negligence.

The upshot is speed limits in my local experience have a lot more to do with the municipality/region's political climate than engineering standards and safety factors. Sometimes I feel like I could safely go 2x, sometimes the limit is 90 km/h on a two-way one lane road with 30 m of visibility where 30 km/h feels like I'm pushing it.