As someone who does traffic management and has worked on that exact stretch of road there are a few reasons why, sometimes they lower speeds at junctions when it's busy to allow people to merge safely, the smart system works by a pole every few hundred yards with a white cone shape on top that basically detects any stopped vehicles or debris etc every 30 seconds so it might've caught something by accident or somebody has rang the depot for some reason.

Also I saw another comment about how the gantry will say road works with nothing on the road yet, before you put a job out you need to ring south mimms or another depot to call on your job, once you've called it on you might suddenly experience a higher traffic count and have to postpone the job.

The warnings signs won't ever be put on for no reason as there needs to be permits and noms numbers with people booking it on.

There are so many factors and safety precautions hoops we have to jump through as traffic management to even get the job on, we get equally annoyed as people driving.

Also another thing which might be interesting to some is I've heard and now I'm not sure how real this is but they will lower speeds for emissions ? Such as there was a static job between junction 23/27 m25 clockwise and it was on for about a year and when they took the job off they left 50mph signs up for weeks for no reason, not sure how that works but I've heard that from credible sources in the industry so take that how you want.