Hmmmm...
They don't look two and a half grades apart on this screen and neither looks like Grade 5. That should be soot and whitewash. How long was the paper developed for? It could be exhausted or cold developer. It doesn't keep. For any kind of testing, fresh developer is essential.
Your testing for personal EI and N development should have taken into account your agitation technique.
Agitation is what it says. There's a very thin layer of developer next to the emulsion which rapidly becomes exhausted, but it still clings to the surface, shielding the film from further action. When you agitate, turbulence should scrub this off and re-mix the whole volume of developer, so that the emulsion has fresh solution next to it. That becomes exhausted in turn and needs more turbulence to re-mix the solution and continue the process.
When you agitate, are you vigorous enough to re-mix the whole volume of liquid?
We standardise on a pattern of agitation (typically for the first 30 seconds and then every half-minute after that) to control the degree of development. Continuous agitation causes the film to be exposed to fresh developer all the time and so less time is needed. Rotary processing typically needs 15% less time but this is always subject to individual experiment. All darkroom processes are subject to individual testing.
If we do not agitate, we run the risk of uneven development, but it is claimed that the most exposed parts of the negative exhaust this thin layer more quickly or more completely and therefore cease to become denser. Meanwhile, the less-exposed parts continue to develop, albeit more slowly. This is considered to be a useful tactic in controlling contrast. Other effects on the transition boundary between light and dark are claimed but that's another matter entirely and needs to be treated separately.