Are you being serious here that this is a problem?
While the gauges are represented a little bit differently (100 inHg max on gauge instead of 75 inHg max), the limits on the manifold gauges that a pilot actually uses are exactly the same, so, using the gauge is exactly the same between the two.
The difference the OP discussed was apparently for other planes, unless Man’0’War is also limited to 38 inHg and should actually go higher.
I just realized, however, I wonder if @JohnnyT5000 was comparing the planes in the Reno Air Race configuration or in Free Flight mode. As I noted above, in Reno Air Race config, all the planes have the same performance. Asobo is hoping to also create a mode for Racing mode that allows users to use the actual plane performance figures eventually, which, currently, can only be seen in Free Flight mode.