Most likely the Red Forge system that you have fitted is a Limitlite system (one of the older versions) and it does not specifically cover GVW like the newer versions with Gross calculations built in. You can confirm this easily - if it has 4 LED's (2 red and 2 green) rather than a digital display you know it is the older version.
At customer request we used to dial down the rear axle overload point in order to cover the gross, as the gross normally goes over before the rear does. When you saw the red light there, and looking from the figures you gave, I would say you were over on your GVW!
The law is that neither an axle, nor the GVW can be exceeded - it is perfectly reasonable to be within both axle plated weights, but over on GVW, as the axles almost always have more tolerance when combined.
As suggested, when measuring, it is true that the rear axle imparts a load onto the front, but that also works the other way round... so if you add front and rear together, that is your gross.
I would say that you are lucky to have got away with that.... at 3810 that would put you over 8% overloaded, and that would normally mean at least a £60 fixed penalty.
If you have any further questions or interest, please feel free to give us a call on 01527 526112 or check out this on our website which gives a bit of a guide (downloadable): http://www.redforge.co.uk/guide/
Phil Bridge - Red Forge.