The only time that is true is if you do very short "trips" (say, less than an hour) in very steady weather. In that case, it will probably mostly be right).
If you don't, and there is a low rolling in (or you rolling into it) you will suddenly have a barometer which reads as if you gain elevation. That's how it works - something you should know by now.
If it was true in general, that frigging plane mentioned earlier wouldn't need to "zero" its barometer with the airport just before landing, because they know what altitude they took off from, so its elevation gain could be read by the barometer. As the bloke who attempted to support you explained: That is simply not good enough.