Don’t want to put gas on fire, but on official documentation it says “Maximum efficiency 95%”.
By that maximum, I, for one, understand that only in certain (ideal) conditions will get to 95%.
In all others, that efficiency could be (and is) lower.
You presented in the first post a particular case where you are using about (if not) the maximum power the MP2 5000 is capable to deliver…
Take a look at this: Multiplus-II 48/5000 efficiency curve - VictronEnergy
Also take into consideration that a grid injection could be subject of rapid power variations and the Cerbo is not always capable of showing you the instantaneous power. You should observe the injection power for a longer period of time and get from there the maximum value and that should be considered when you are computing the efficiency.