Years ago using Agfa's papers, I was surprised not to be able to get a very hard grade on them.
A phone call to an Agfa rep gave me the answer : the grade 5 can only been got at by using a very dense magenta filter (the filters Agfa made for their papers are quite different from their Ilford counterpart).
They guy was kind enough to send me a set of Agfa filters. If you place the #5 Ilford next to the #5 Agfa, the difference is obvious. This may explain the results you've got.
Just my 2cents