I should have considered the transmission power limits of the B versus the much more powerful A class AIS units. I have been looking at Marine Traffic and Ship Finder for some time and there are some anomalies, for sure. Most recently, I was seeing on Marine Traffic a tanker anchored outside Portsmouth, near to the Isles of Shoals. There was no sign of it on Ship Finder.
Both are receiving my transmissions, so why one shows a target and the other does not, I do not know.
No, this is not a business interest of mine. Just a hobby.