Oil vapour has a lot more pipe to travel to get to mechanical boost gauge.On the mechanical boost gauge side theres a restrictor ,like 1mm diametre ,to avoid flickering of gauge when the car is not boosting.Oil vapour is harder to get through !Yes they both have inline filters.The other thing is ,an electric boost gauge costs 5x more than a mechanical one and I feel the mechanical is more accurate.
Regarding the boost pressure,mine is still the pre facelift!I have the actual map for the car and standard it does 1.25 with peaks of 1.4 and never falls below 0.9 bar on full acceleration.I can confirm that in practise,the boost readings are an exact match to these figures.I remapped mine a while ago and boost is higher throughout the revs but boost peak is only slightly altered (1.7).Yours should have pretty much the same figures,as on the facelift ,Bmw fitted an even bigger turbo and injuectors,so they didn t need to raise boost at all.