In my experience this applies to "bars" of all Gs, I've been in many places with full bars and unable to load anything. Seems that bars don't indicate backhaul bandwidth or congestion.
It's not even true signal strength. Certainly on 2G (I'd have to double check on later technologies) the radio can inform the UE to treat the recieved signal level as being an arbitrary amount better than actually measured.
This applies to signal level as reported to the user as well as internal metrics, eg handover (is this other cell better, should I switch to it?).
