There is no "speed limit on the motorway" except the default 70mph. When variable limits are in use, they can and do vary on different sections of road. The 1hr 27 mins applies only to the gantry where you were photographed, and it is entirely possible that it had remained unchanged for a long period while those before and/or after it varied.
I ask again, did you see the limit on that particular gantry change as you approached?
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Ok thanks and that explained it really well.
I am not sure if the speed limit changed on that particular gantry.
My issue is the volume of different variable limits applied through out the section of the motorway - its like they were put in place to deliberately trick users with the constant changes as each gantry was approached.