Well, who knows. I think Atina is right with her guess, that it could be a delay of the alarm. Moreover, as we all know, houses often go within seconds. And even if you know how to trick the refresh limit (which is possible - but not recommendable), it could be that a Bellisseria-home shows up and finds a new owner in the 20 (or 60-70) seconds while your add on didnt make a refresh (*).
Personally I now only log in on the land-selection page, when I have to do some PC-work anyways. I move the browser window on my second screen and can quite quickly see if there is some Bellisseria home. But even then no guarantee, that Im quick enough (see *). Thats why I log off from the sl-homepage, as soon as I leave the PC.