J.B.
Sub chorionic bleeds are very common. The placenta is so small now that the particular spot where it detached won't re-attach, but the rest of it will grow attached. Picture it like if you put a piece of tape on something but have bubble in one spot, except that this bubble is filled with blood. Some clots are large enough to cause complications, but most are just a temporary annoyance that goes away on its own.
I had a sub-C bleed in my 2nd pregnancy and swore that I was having a miscarriage at 10 weeks (it was A LOT of blood and tissue and I was bleeding for over an hour) and he was totally fine.
Don't worry about the measurements. They're just estimates. The angle of the wand, etc. can change the measurements a lot.