What is the purpose of AY's 8kg limit on cabin baggage?

AY, together with BA, IB, U2 and several other airlines, allows the largest size of cabin baggage, 56x45x25cm. As far as weight is concerned, BA allows 23kg and U2 allows unlimited weight as long as the passenger can lift it. For the same size limit, why is AY's weight limit one third of BA's?

Imposing size limits makes a lot of sense, because there is limited space in the overhead lockers. But there is no more sense in imposing a low weight limit on cabin baggage than there is in imposing a weight limit on passengers' bodies or the contents of passengers' pockets. Based on AY's policy, a 40kg child with 10kg cabin baggage (50kg total) will be too heavy, yet an obese 120kg adult with 8kg cabin baggage (128kg total) will be within the limit. It's absurd and defies logic. What difference does it make to AY whether weight is in the passenger's cabin baggage as opposed to in the passenger's body or in the passenger's pockets?

I also observed poor knowledge by AY staff of the 56x45x25cm size limit. My girlfriend's and my 56x45x25cm cases didn't fit in the overhead lockers of an Embraer ERJ-190LR from HEL to TLL or of an ATR 72-500 from TLL to HEL. Both times, the cabin crew stated that they were larger than allowed by Finnair, which is false. The same cases fit into the overhead lockers on BA's Embraer ERJ-190SR aircraft. Why does AY not clarify on its web site that regulation-size cabin baggage might have to go into the hold on some aircraft?

When boarding from HEL to LHR, the gate staff doubted whether my girlfriend's 56x45x25cm case was within these dimensions and asked her to put it into the measuring frame. The case has only one handle, which is at the top when upright at 56cm high, whereas the measuring frame was 45cm high, 56cm wide and 25cm deep. This meant that there was no way of holding the handle while lowering it into the measuring frame, and the handle would eventually be hidden within the measuring frame, making it impossible to pull the case back out. I therefore refused to drop it all the way in, and although the gate staff, despite complaining, eventually agreed that it was the correct dimensions. I'm all in favour of measuring frames, particularly by airlines that allow this maximum size, but why does AY create measuring frames in an unusual orientation whereby the only handle might become hidden in cabin baggage's usual orientation, making it impossible to pull the case back out?