It's not uncommon for manufacturers to quote a maximum standby time that's quite unachievable in reality: they presumably test (or calculate) the lifetime with screen off, data off, immobile in a maximum signal strength area, no interactions with anything except minimal network handshakes. But that's got nothing at all to do with the lifetime when actually used.
To illustrate, the "official" figure for my old HTC Desire is "up to 340/360 hours", i.e. 14-15 days. When the battery was new, and running a more efficient ROM than the HTC one, I think that was theoretically possible, based on overnight consumption when the phone was in deep sleep (2% drop over 8 hours was achieved). But 3-4 hours of use with the screen on will run it flat, so in reality most owners charged them daily. So there's nothing unusual about a huge gap between theoretical standby figures and real use.
A phone going flat overnight isn't right though, unless in fringe coverage. Dropping and reconnecting to the network constantly will however run a phone down really quickly, as will constant switching between 2G and 3G.