Journals with title "Bulletin"
Don't read too much into the nomenclature of journals. Some of them are very old and the names are just traditional. The purpose of some may have evolved over time a bit but not the names. Each different society may use its own naming conventions as may each commercial publisher. 

Nobody has looked at the whole field and tried to rationalize it. And even if they tried, the traditional names would stick.

But Bull. AMS is explicitly for expository articles, not the latest hot topic research. Therefore its "reputation" is different in kind from that of Trans. AMS, though both are high quality.