I would need to speak to Buddha about it if I’m right or wrong but this is how it is related to my experience and experience of others I’m trying to guide into Nibbana.
The first entry of Nibbana can be very potent before it stabilizes itself as Sotapanna, Sakadagami etc. One person can have so strong samadhi that it can actually make one think that one attain a higher stage of what he really attained. To take the example of suttas see how many Sotapannas have different reactions in relation to stream entry. Sariputta saw “Deathless” when encountering first stream-entry, he would recognize it quickly or had very strong experience while others can’t deny the truth of Nibbana and Buddha teachings but they experience is not that deep or intense as some of the others.
My idea behind it (which I would like to ask Buddha as this is just in terms of simple observation) I personally had very strong “hunger” for Nibbana without really knowing what exactly I was looking for before I found it. Imagine this like eating different things but it does not really feel like it satisfies you, you can feel you eaten but it’s not just “it”. People who happen to have some spark or possibility to attain Nibbana but do not have that “fire” or “hunger” I see tends to experience Nibbana rather slowly or like Lal explained in his post that sometimes it takes weeks before one sees it. If the pressure is strong and a person is looking for Nibbana without even knowing what exactly he is looking for I think he can expect very good “experience” of Nibbana even before attain Anagami or Arahant stage. A person who is just “interested” or “curious” would rather have small or almost to none experience beside clarity of insight into Nibbana.