rabbiofberlin & sjsinnyc,
The answer to both of your questions is one in the same.
Some times a Rov can hold A & understands that there is room for another Rov to hold B – he simply does not agree with that p’sak.
However, sometimes (& this is the case with the Flatbush eiruv) the Rabbonim who hold A are of the opinion that B is completely & totally erroneous & therefore there is no room to be lenient whatsoever.
I myself saw signs up signed by two of the most prominent poskim in America as well as Rav Elyashiv Shlit”a stating that anyone who carries in the eiruv is a “mechalel shabbos b’farhesia”.
So again, I’m a nobody & I can’t tell you not to rely on your Rabbi, but the aforementioned Rabbonim are of the opinion that your Rabbi is making a MISTAKE, they don’t merely disagree.