Scottish "70mph" posted speed limits

I have noticed it on slip-roads onto dual-carriageways / motorways. In that circumstance it perhaps just clarifies that once you are on the slip-road, it is a 70 limit (for cars / small vans / some buses). It seems to hold that on direct entry to a dual-carriageway (a roundabout perhaps), it is a national speed limit sign that is displayed. Similarly, if a stretch of dual-carriageway / motorway is an enforced 40 or 50 for all vehicles, the point where this restriction is lifted shows a national speed limit sign.

I would suggest that the effective speed limit for a goods vehicle over 7.5 tonnes would be the speed limit for the road the slip-road joins on to. Of course in Scotland this depends if you are joining a dual-carriageway or a motorway.

Not presenting the above as gospel, but it seems to be logical.