Front Gauge is - as you say fill pressure of the bottle.
Rear Gauge is the Regulator gauge. As your rifle is Sub 12, it's factory set via the Reg Adjuster ( just behind the trigger ) and the reading is the pressure that the regulator is set too. Most Sub 12 rifles run lower than high power / FAC - these run sometimes well above 100 and I have set my Crown up to 170 at times. FX use the same gauges for both high power and sub 12.
All is as it should be. ?
Daystate / Brocock and others all do the same.
Optimum pressure, can vary slightly by rifle / settings / would suggest you check your FPS - once you know your FPS then you know how your set. If its factory set, works and your knocking down targets, best advice is don't tinker.
Hope that helps.
G