Is this the "Your online application has been received at the UK Decision Making Centre. Your application is currently being prepared for consideration by an Entry Clearance Officer. email?
Yes.
Basically, in other countries, you submit the documents at the same time as giving biometrics, so the count starts from that date, because that's the date UKVI receive your documents.
However, in the US, you give biometrics first, then mail your documents, so they don't start the count until they receive your documents in Sheffield... because they have no control over how long it takes you to mail your documents to them.
For example, the visa decision might be made 60 working days after your biometrics, but they might not have received your documents until working day 20... which means actually, it only took them 40 working days to make a decision... the other 20 working days would have been your fault for not sending the documents earlier.