What are likely reasons for being asked by UK immigration to fetch my email flight confirmation despite API?

I (a south Slavic looking Swede) entered the UK at Luton from Larnaca the other day (black t-shirt+white 3/4 trousers+rucksack). Went to the only manned non-EU booth, which, unlike the EU section, was empty.

The convo went like this (with IO being mildly grumpy throughout):

Me (handing over my ID card): Good morning Sir.

(pause)

IO: Do you have a passport?

Me: No, how so?

IO: Any other photo ID?

Me: Er, nothing photographic I'm afraid (checking my wallet)

IO: Can I see your wallet?

Me: Sure, I have some bank cards in it

IO: Yeah that'd be good

(I hand over one of my debit cards)

IO: So where did you fly from?

Me: Larnaca

IO: OK...you have the boarding pass do you?

Me: No, left it on the plane.

IO: Well got it on the phone don't you?

Me (lightly smiling): You mean this (showing my Nokia 1112)?

Me: Sorry, but what's the matter either way?

IO: Well I need to know where you flew from.

Me: Should be in your API records, no?

IO: You know about that don't you?

Me (humbly): I have an interest in immigration matters.

IO: Well you don't know enough to know what I'm doing.

Me: No no, that's all fair, but I mean, did the airline not transmit the data properly or something?

IO: No, you're overthinking it my friend.

IO: So what happened to the boarding pass? Why did you discard it?

Me: I always leave it on the plane.

IO: Where did you leave it?

Me: In the pocket in front of my seat.

IO: OK, well I need proof of where you flew from. What can we do?

Me (thinking): Is there Wifi here?

IO: There is

Me: I could log onto my email and show you the email booking confirmation

IO: You get to it!

(I take out the laptop and start it)

(I now realise IO finds something off about me)

Me: Excuse me, may I ask what's so strange about me?

IO: No you can't

(pause whilst I'm logging onto the Wifi)

IO: So do you live in the UK?

Me: No, Zurich, Switzerland

IO: OK, and how did you become a Swedish citizen?

Me: Born and bred, I'm from Stockholm, only I moved after secondary school for my higher studies

IO: But you studied in the UK? Or lived there?

Me: No. Why, because of my accent? No, that's just how I learned the language.

IO: Whatever, just get that thing for me yeah?

Me: I'm right on it Sir

(showing email confirmation)

Me: There you go. So what now?

IO: Alright, you go on then, thanks.

Me: Take care.

I suspect ethnic profiling, but why would an email flight booking confirmation out of all things help with that? Surely I could easily fake it if I wanted to?