I think they do that because the disappearing twin can occur. Usually at 10 weeks+ it's safer and already gone if that occurred. My uncle announced twins years ago based on the early ultrasound much to his wifes dismay. They ended up with one, the other disappeared.
That's why as eager as I am, I'm waiting a few more weeks since I really want twins. I don't want that to happen to me either and be disappointed that one disappeared.
Update! I called my doctors office back and explained my worries and they agreed that they thought I would get in sooner so they called themselves and made them reschedule me for sooner.
I had a scan at 5 weeks and could see the gestational sack and a heartbeat. I had spotting similar to you so was checked out by the early pregnancy clinic because I too had miscarried a few months before.
Exactly! They can see something at this point.. I managed to get a scan scheduled for Tuesday, so only a few days of waiting.