Is this a new gauge installation? If "no", then did this gauge previously read more in line with the HD manual? If it used to read per the manual and is now reading high you have a problem that needs addressing.
At best, I would say that all gauges that we have whether they are HD original equipment or aftermarket and whether they are pressure or temp should be used a "trend" indication.
If it is a new installation, then this gauge is reading higher than normal and that would your "norm". You should monitor your "trend" indications from its "norm".
You could ask the dealer that you purchased it from to give you another.