According to Alton Brown, Sir Thomas Lipton came up with the BOP/OP/FTGFOP grading system, which was standardized. Different leaf sizes pass through mesh screens with standardized weaves, etc. etc. I wouldn't suggest quoting good ol' AB's tea episode much, but he's probably correct history-wise. Lipton was very involved in the Ceylon tea business, so it started and spread from there.
I think attempts were made by some to come up with a grading system to Chinese and Japanese teas, but it never really caught on to the same extent as Lipton's.
As to why some vendors don't list leaf grade, my guess would be that most of their target audience wouldn't know what to do with it. You could probably find out if you emailed them.