I can see how you can think one adult and three kids, another adult and three kids, two adults and one kid, leads to equal split. The two missing kids in the third party equates to one adult.
However when one of those parts of the overall party doesn't consume two potentially expensive items that the other adults do then the basic simple split no longer works.
Assume 1A meal = £10, 1C Meal = £5, Wine = £15 and Cheeseboard = £6
Poster = Adult meal @ £10, plus three kids meals @£15, one third wine @£5 and one third of cheeseboard = £2. Total = £32
Joan = 1 Adult meal @£10, plus three kids meals @£15 = £25
Couple
= 1 Adult meals @£10, plus one kids meal @£5, one third wine @£5, one third cheeseboard @£2 = £22
plus one adult meal @£10, one third wine @£5 and one third cheeseboard @ £2 = £17
couple in total = £39
Total for meal in this example is £96 (if my maths is correct) and a three way split would be £33 each - Joan would be overpaying by £8, the poster would be saving £1 and the couple would be paying £6 more but that £6 purchases a meal plus cheese & wine.
If it had been split 4 ways it would be £24 each with the couple paying £48. So Joan would be paying nearer her share, the poster would be benefiting by £8 and the couple would be overpaying £3 each (£6 as a couple)
Put another way - the couple's share of the bill was 40%, the posters was 33% and Joan's only 26%. This is only one example obviously the costs could all be totally different but the end result would be the same, just the scales would perhaps differ - Joan definately had the smallest bill. The poster probably did have about a third of the bill and therefore would pay a third even if it was split exactly so possibly doesn't see what the fuss is. The big discrepancy will be between the couple and the single with no extra items in her choices.
Maths isn't my strong point, so I may be wrong here but I think the basics are right. I hope so anyway or I've made a complete fool of myself.
I wouldn't mind an even split but that's because I'm in a fortunate financial position and things tend to even out over the years - my friend forked out over £30 for all of us for additional add ons a day out recently plus she then paid for lunch as she was first in the queue and I was sheparding six kids towards the tables. I thanked her and she wouldn't take the money but we know I will do it next time. With other friends who I know are on a bit of a tighter budget I will make sure I am at the front of the line and will pay and not expect to be paid back. Usually I just say, oh you get it next time (which they may or may not do depending on how it all works out)