Filtering account issue

[aga]

I wanted to list transactions for a particular payee, Plusnet, but am 
puzzled by the results.
If I enter into the filter box a character sequence that is not present 
in the particular account displayed, then no transactions are listed, as 
expected.  If instead, I enter 'p', all transactions are still 
displayed, also with 'pl' and 'plu' and 'plus'.  it's only when I enter 
'plusn' that the filtering works.  Similarly, 'u' and 'us' don't filter, 
but 'usn' works.
Is there a reason why, I wonder?

Allan